Section 104 Pool Calculator
HMRC requires UK investors to use the Section 104 poolto calculate the average cost of shares purchased over time. Each time you buy more of the same share, the pool's total cost and quantity increase. When you sell, the allowable cost is calculated proportionally from the pool.
Enter your purchase history below to build your pool and see how the average cost changes with each acquisition. Then enter a disposal to calculate your gain or loss.
Purchases
Disposal
How the Section 104 Pool Calculator Works
This calculator uses precise decimal arithmetic(the same library as our main CGT engine) to avoid floating-point rounding errors that can affect gain calculations. Each purchase you add increases the pool's total shares and total cost. The average cost per share is recalculated after each addition.
When you enter a disposal, the calculator deducts the proportional cost from the pool. The allowable cost for the disposal is: (shares sold ÷ total pool shares) × total pool cost. The gain or loss is simply the proceeds minus this allowable cost.
Worked Example
Imagine you bought 100 shares at £2.00 (cost: £200) and later 50 shares at £3.00 (cost: £150). Your pool is: 150 shares, £350 total cost, £2.3333 average. If you sell 60 shares at £4.00:
- Proceeds: 60 × £4.00 = £240
- Allowable cost: (60 ÷ 150) × £350 = £140
- Gain: £240 − £140 = £100
- Remaining pool: 90 shares, £210 cost
For the full rules on how the Section 104 pool interacts with same-day and 30-day matching, read our comprehensive Section 104 guide.
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