How to Calculate Weighted Average Cost for Inventory
Divide total cost of goods available by total units available. Formula, COGS and ending inventory examples, periodic vs perpetual, FIFO compared.
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A workshop buys the same cordless drill three times in one year, at $18, $21 and $24. Ask what a drill costs and the honest answer is $21.30, not the $21.00 you get by averaging three price tags.
The difference is quantity. Two hundred units came in at $18 and five hundred at $21, so the cheap lot and the mid lot are not equal partners. Get this wrong and both your profit and your balance sheet inherit the error.
This guide works one inventory scenario all the way through: the cost per unit, cost of goods sold, ending inventory, periodic against perpetual, and how the answer compares to FIFO and LIFO. The weighted average cost calculator and the weighted average inventory calculator run these numbers for you, and the weighted average calculator handles the general case.
What Is Weighted Average Cost in Inventory?
Weighted average cost assigns every unit in stock the same blended cost, calculated by dividing the total cost of goods available for sale by the total units available. No unit is treated as older or newer than another.
How the Weighted Average Cost Method Works
Pool every unit you could have sold, pool every dollar you paid for them, then divide. That single figure costs every sale and values everything left on the shelf.
Why Inventory Costs Are Weighted by Quantity
Because a 500-unit purchase affects your real cost far more than a 50-unit top-up. Averaging price tags pretends the two orders were the same size.
When Weighted Average Cost Is Used for Inventory
It suits interchangeable goods where one unit is genuinely indistinguishable from the next. Fuel, screws, grain, raw chemicals, commodity components. IAS 2 Inventories permits it alongside FIFO for exactly those cases.
Weighted Average Cost Formula for Inventory
Weighted average cost per unit equals total cost of goods available for sale divided by total units available for sale. Both halves include beginning inventory plus every purchase made during the period.
Total Cost of Goods Available for Sale
Beginning inventory at cost plus every purchase at cost. Here that is $3,600 plus $10,500 plus $7,200, giving $21,300.
Total Units Available for Sale
The same rows counted in units instead of dollars. 200 plus 500 plus 300 gives 1,000 units.
Weighted Average Cost Per Unit
$21,300 ÷ 1,000 units = $21.30 per unit
How to Calculate Weighted Average Cost Step by Step
Start with beginning inventory, add every purchase, total the cost column, total the unit column, then divide cost by units. Five steps, and the fourth is the one people skip.
Step 1: Determine Beginning Inventory
Pull the closing figure from last period, in both units and dollars. Our workshop opened with 200 units carried at $18.00, worth $3,600.
Step 2: Add Inventory Purchases
List each purchase as its own row with its own unit cost. Freight and duty belong in these costs, which is where many first attempts quietly go wrong.
Step 3: Calculate Total Inventory Cost
Add the dollar column. Beginning inventory belongs in this total, not beside it.
Step 4: Calculate Total Units Available
Add the unit column separately. Check it against your stock count before going further, because a wrong denominator poisons everything downstream.
Step 5: Calculate the Weighted Average Cost Per Unit
Divide total cost by total units. Carry four decimal places internally and round only when you report.
Interactive Three cost layers, one blended cost
Edit any quantity or unit cost. The bar shows how much of your total inventory cost each layer
owns.
- Beginning inventory —
- March 8 purchase —
- June 14 purchase —
Weighted average cost per unit $21.30
Averaging the three sticker prices gives $21.00. Weighting by quantity gives $21.30, because the 500-unit March lot carries half the units on the floor.
Weighted Average Cost Inventory Example
Beginning inventory of 200 units at $18.00 plus purchases of 500 at $21.00 and 300 at $24.00 gives 1,000 units costing $21,300. The weighted average cost is $21.30 per unit.
Beginning Inventory and Purchase Costs
| Cost layer | Units | Unit cost | Layer cost |
|---|---|---|---|
| Beginning inventory | 200 | $18.00 | $3,600 |
| March 8 purchase | 500 | $21.00 | $10,500 |
| June 14 purchase | 300 | $24.00 | $7,200 |
| Goods available for sale | 1,000 | — | $21,300 |
Calculating Total Cost and Total Units
$21,300 across 1,000 units. Both totals come from the same three rows, which is why a missing purchase breaks them together rather than separately.
Calculating the Average Cost Per Unit
$21,300 divided by 1,000 gives $21.30. Note that it sits above the $21.00 price average, because 800 of the 1,000 units cost more than $18.00.
Applying the Average Cost to Inventory
Every unit now costs $21.30, whichever lot it physically came from. That is the method’s central claim, and the reason it only suits interchangeable goods.
How to Calculate Cost of Goods Sold Using Weighted Average Cost
Multiply units sold by the weighted average cost per unit. With 700 units sold at $21.30, cost of goods sold is $14,910. No lot tracking is required because every unit carries the same cost.
Calculate Units Sold
Units sold equals goods available minus units remaining. Our workshop ended with 300 units, so it sold 700.
Apply the Weighted Average Cost Per Unit
One multiplication. The same $21.30 costs the first unit sold in March and the last one sold in November.
Weighted Average Cost of Goods Sold Formula
COGS equals units sold times weighted average cost per unit. That is the entire formula.
Worked COGS Example
700 units × $21.30 = $14,910
How to Calculate Ending Inventory Using Weighted Average Cost
Multiply units remaining by the same weighted average cost per unit. With 300 units left at $21.30, ending inventory is $6,390, and it must add back to goods available alongside cost of goods sold.
Calculate Units Remaining
Count them, do not derive them. A physical count that disagrees with your records is information, not an inconvenience.
Apply the Weighted Average Cost Per Unit
The same $21.30 again. Using a different rate for the balance sheet than the income statement is a reconciliation failure waiting to happen.
Ending Inventory Formula
Ending inventory equals units remaining times weighted average cost per unit.
Worked Ending Inventory Example
300 units × $21.30 = $6,390 → $14,910 + $6,390 = $21,300
Periodic vs. Perpetual Weighted Average Cost
Under a periodic system, one average is calculated for the whole period. Under a perpetual system, a moving average is recalculated as purchases occur. Identical transactions can therefore produce different cost of goods sold.
How the Periodic Weighted Average Method Works
Wait until the period closes, pool everything, divide once. Simple to run, and it cannot tell you your margin in July.
How the Perpetual Moving Average Method Works
Recalculate the average the moment new stock arrives, then charge sales at whatever the current average is. Every modern inventory system does this automatically.
When the Average Cost Is Recalculated
Perpetual
Opening 200 @ $18 Mar 8 buy 500 @ $21 Mar 20 sell 400 units Jun 14 buy 300 @ $24 Aug 5 sell 300 units
Recalculates at every purchase. Three different averages were in force during the year.
COGS $14,678.57 Ending $6,621.43
Periodic
Opening Mar 8 buy Mar 20 sell Jun 14 buy Aug 5 sell Period end
Recalculates once, after the last transaction. One average of $21.30 covers the whole year.
COGS $14,910.00 Ending $6,390.00
Why the Results Can Differ
Because timing matters. The perpetual system charged March’s sale at $20.14, before the expensive June stock existed. The periodic system retroactively lets June raise the cost of a March sale.
Weighted Average Cost Example Under a Periodic System
One average of $21.30 covers the whole period. With 700 units sold, cost of goods sold is $14,910 and ending inventory is $6,390. The two total $21,300, the goods available for sale.
Calculate Total Goods Available
1,000 units costing $21,300, exactly as calculated above. Nothing about the sales pattern changes this total.
Calculate Weighted Average Cost
$21,300 divided by 1,000 gives $21.30. Calculated once, after the last transaction of the year.
Calculate Cost of Goods Sold
700 units times $21.30 gives $14,910, charged to the income statement.
Calculate Ending Inventory
300 units times $21.30 gives $6,390, carried on the balance sheet.
Weighted Average Cost Example Under a Perpetual System
The average is recalculated after each purchase. Sales are charged at the average in force that day, producing cost of goods sold of $14,678.57 and ending inventory of $6,621.43 from the same transactions.
Calculate the Initial Average Cost
Opening balance of 200 units at $18.00. Until something arrives or leaves, that is the average.
Recalculate Average Cost After a Purchase
The March purchase lifts the balance to 700 units and $14,100, so the average becomes $20.1429.
Apply the Current Average Cost to Sales
Interactive Play the perpetual ledger forward
Every purchase resets the average. Every sale is charged at whatever the average happens to be
that day.
Opening balance of 200 units carried in at $18.00 each.
Units on hand 200
Inventory value $3,600.00
Moving average cost $18.0000
COGS booked so far $0.00
Step 1 of 5
Update Inventory After Each Transaction
Each sale reduces both units and value by the average rate, leaving the average itself unchanged. Only purchases move it. That property confuses people and is worth reading twice.
Weighted Average Cost vs. FIFO and LIFO
On identical data, FIFO reports the lowest cost of goods sold, LIFO the highest, and weighted average cost lands between them. All three reconcile to the same $21,300 of goods available for sale.
Interactive Same 1,000 units, three costing methods
700 units sold in every case. Only the cost attached to them changes, and it changes by $1,500.
First in, first out
The oldest units leave first, so the 200 cheapest units at $18.00 go out before anything else.
Cost of goods sold $14,100.00 200 units at $18.00, then 500 units at $21.00
Ending inventory $7,200.00 300 units at the newest $24.00 cost
Lowest COGS, highest ending inventory, highest reported profit while prices rise.
Reconciliation: $14,100.00 plus $7,200.00 equals $21,300.00, the goods available for sale. Every method must land here.
Weighted average cost
No unit is older than another. Every unit carries the same blended $21.30 cost.
Cost of goods sold $14,910.00 700 units at $21.30
Ending inventory $6,390.00 300 units at the same $21.30 cost
Sits between the other two, by construction rather than by coincidence.
Reconciliation: $14,910.00 plus $6,390.00 equals $21,300.00, the goods available for sale. Every method must land here.
Last in, first out
The newest units leave first, so the 300 expensive units at $24.00 go out immediately.
Cost of goods sold $15,600.00 300 units at $24.00, then 400 units at $21.00
Ending inventory $5,700.00 100 units at $21.00 plus 200 units at $18.00
Highest COGS, lowest ending inventory. Permitted in the US, prohibited under IFRS.
Reconciliation: $15,600.00 plus $5,700.00 equals $21,300.00, the goods available for sale. Every method must land here.
Weighted Average Cost vs. FIFO
FIFO sells the oldest units first, so the $18.00 lot leaves before anything else. COGS of $14,100 against our $14,910, and $810 more profit reported on the same sales.
Weighted Average Cost vs. LIFO
LIFO sells the newest units first, giving COGS of $15,600. It is permitted under United States tax rules but prohibited under IFRS, which is why it barely exists outside the US.
How Each Method Assigns Inventory Costs
FIFO and LIFO track cost layers and care which unit moved. Weighted average cost refuses to distinguish them at all, which is precisely why it needs no lot tracking.
How Purchase Price Changes Affect COGS and Ending Inventory
Advantages and Limitations of Weighted Average Cost
The method is simple, smooths volatile purchase prices and needs no lot tracking. It also hides genuine cost differences, struggles with highly variable prices, and depends entirely on accurate quantities.
Advantages of the Weighted Average Method
Simplifies Inventory Cost Tracking
One rate covers everything. No lot numbers, no layers, no arguments about which pallet shipped.
Smooths Purchase Price Fluctuations
A single expensive order does not spike reported margin the way FIFO or LIFO can.
Works Well With Interchangeable Inventory
When units are genuinely identical, pretending to track them individually is bookkeeping theatre.
Limitations of the Weighted Average Method
Can Hide Significant Price Differences
Our $18 and $24 lots both report as $21.30. A buyer who negotiated hard gets no visible credit for it.
May Be Less Useful With Highly Volatile Costs
Current inventory guidance commonly notes that weighted average cost smooths price fluctuations, while highly variable purchase prices can make the method less representative of individual inventory batches.
Requires Accurate Inventory Quantities and Costs
The denominator is a physical count. If the count is wrong, so is every downstream figure, as what a weighted average is explains in other contexts.
Common Mistakes When Calculating Weighted Average Inventory Cost
Six mistakes cause most wrong answers: dividing by purchase count, omitting beginning inventory, using price instead of total cost, applying the wrong average to sales, mixing the two systems, and rounding too early.
Using the Number of Purchases Instead of Total Units
Dividing $21,300 by three purchases gives $7,100, which is not a unit cost on any planet. The denominator is units.
Forgetting Beginning Inventory
Omitting the opening 200 units gives $17,700 over 800 units, or $22.13. That overstates cost of goods sold by more than $580.
Using Purchase Price Instead of Total Inventory Cost
Averaging $18, $21 and $24 gives $21.00 rather than $21.30. The gap is small here and large whenever lot sizes diverge, as weighted average vs simple average shows across other fields.
Applying the Wrong Average Cost to Units Sold
Under a perpetual system, charging a March sale at the December average is a real and common error. Use the average in force on the transaction date.
Mixing Periodic and Perpetual Calculations
Pick one system and stay in it for the whole period. Half a ledger of each reconciles to nothing.
Rounding the Average Cost Too Early
Rounding $20.142857 to $20.14 before multiplying by 400 units costs you $1.14 immediately. Round at the end, never in the middle.
How to Check a Weighted Average Inventory Calculation
Verify total units against a physical count, verify total cost against purchase records, recompute the cost per unit, and confirm that cost of goods sold plus ending inventory equals goods available for sale.
Verify Total Units Available
Compare 1,000 units against your stock records and receiving documents. A mismatch means a delivery is missing or duplicated.
Verify Total Cost Available
Tie $21,300 back to supplier invoices, including freight. Ledger balances that disagree with invoices are worth chasing before anything else.
Check the Average Cost Per Unit
Recompute $21,300 divided by 1,000 independently. The weighted mean calculator and the weighted percentage calculator both print the two totals separately for this kind of check.
Reconcile COGS and Ending Inventory
$14,910 plus $6,390 equals $21,300. If your two figures do not add back to goods available, stop and rebuild rather than adjusting one of them.
Check for Rounding Differences
Small residuals are normal under a perpetual system. Anything above a few cents means you rounded a rate rather than a result. Spreadsheet users should see the Excel and Google Sheets guide for keeping full precision in a live file.
Frequently Asked Questions About Weighted Average Inventory Cost
What is the weighted average cost method?
It is an inventory costing method that assigns every unit the same blended cost. You divide the total cost of goods available for sale by the total units available, then use that one rate for both cost of goods sold and ending inventory. It suits interchangeable goods and is permitted under both IFRS and US GAAP.
What is the formula for weighted average inventory cost?
Weighted average cost per unit equals total cost of goods available for sale divided by total units available for sale. Both halves include beginning inventory plus all purchases in the period. In our example that is $21,300 divided by 1,000 units, giving $21.30. Freight and duty belong inside the cost figure.
How do you calculate weighted average cost per unit?
Add the cost of beginning inventory to the cost of every purchase, add the units the same way, then divide. $3,600 plus $10,500 plus $7,200 gives $21,300 across 200 plus 500 plus 300 units. Dividing returns $21.30. Keep four decimal places while you work and round only in the final report.
How do you calculate COGS using weighted average cost?
Multiply units sold by the weighted average cost per unit. Selling 700 units at $21.30 gives cost of goods sold of $14,910. Under a perpetual system you instead use the average in force on each sale date, which produced $14,678.57 from the same transactions. Both approaches are legitimate, but they are not interchangeable mid-period.
How do you calculate ending inventory using weighted average cost?
Multiply the units still on hand by the same cost per unit. Our 300 remaining units at $21.30 give $6,390. The reliable check is that cost of goods sold plus ending inventory must equal goods available for sale, so $14,910 plus $6,390 returns exactly $21,300.
What is the difference between periodic and perpetual weighted average cost?
Periodic calculates one average after the period closes. Perpetual recalculates a moving average every time stock arrives, then charges sales at the current rate. Our data gives $14,910 of cost of goods sold under periodic and $14,678.57 under perpetual, a $231.43 difference caused purely by timing rather than by any error.
Is weighted average cost the same as moving average cost?
Moving average cost is the perpetual version of weighted average cost, so the terms overlap without being identical. When someone says weighted average cost without qualification they usually mean the periodic single-average calculation. Inventory software almost always implements the moving average, which is worth confirming before you reconcile anything by hand.
What is the difference between weighted average cost and FIFO?
FIFO assumes the oldest units sell first and tracks cost layers. Weighted average cost blends every unit into one rate and tracks nothing. On our rising prices, FIFO reported $14,100 of cost of goods sold against $14,910 under weighted average, so FIFO showed $810 more profit and a higher closing inventory value from the same physical stock.
Can weighted average cost be used when inventory prices change?
Yes, and price movement is exactly why it exists. The method absorbs each new purchase price into a blended rate, which smooths the volatility that FIFO and LIFO pass straight through to margin. The caution is that very large swings make the single rate less representative of any actual batch you hold.
Does weighted average cost affect COGS and ending inventory?
Yes, and always in opposite directions. Goods available for sale is fixed at $21,300, so every dollar you do not charge to cost of goods sold stays on the balance sheet. That is why switching methods changes reported profit and inventory value simultaneously, and why accounting standards require you to apply one method consistently.
Related Inventory Calculators
Related Calculators
That workshop now costs every drill at $21.30 and reconciles to the penny. The fix was not clever accounting. It was counting units before averaging prices. More questions about weighting are answered case by case, the weighted average calculator prints the weighted sum and total weight side by side, and how to calculate a weighted average covers the same arithmetic outside the stock room. Which layer is hiding in your closing count?