Your purchases
Averaging the prices gives the wrong answer
Average cost is total spent ÷ total units, not the mean of the prices you paid. Buy 10 units at 100 and 90 units at 50 and the prices average to 75, while your actual average cost is 55. The number is pulled towards wherever the quantity is.
Quantity, not price, decides how far the average moves
Adding a small position after a fall barely shifts the average. If you already hold 100 units, buying 10 more gives the new price less than a tenth of the weight. Moving the average meaningfully takes a purchase comparable to what you already hold — and that scales the risk up by the same factor.
A lower average is not the same as a smaller loss
Buying more on the way down lowers your break-even price, not the money you are down. Since the total invested has grown, the same percentage fall now costs you more. This tool works out the average; whether the purchase was wise is a separate question. Fees and tax are not included, so your true break-even sits slightly above the figure here.
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Frequently asked questions
Q. Why is my average cost not halfway between my two buy prices?
Because the two buys were for different quantities. The average sits closer to the price at which you bought more units, in exact proportion to those quantities.
Q. Does selling part of the position change the average?
Under the weighted-average method, no — selling reduces the units but leaves the average cost per unit as it was. Some tax regimes require FIFO instead, which does change the cost basis of what remains.
Q. How do I include fees?
Add them into the price you enter, spread over the units in that purchase. Kept outside, the average shown will be slightly optimistic.