The formula
How to calculate stock loss
A loss and the recovery it demands are not the same size. Falling 35% requires a 54% gain to get back to where you started, and that asymmetry is the most important thing this calculator shows.
The gap widens sharply as losses deepen: down 20% needs 25% back, down 50% needs 100%, down 80% needs 400%. The arithmetic is unforgiving because the gain is calculated on the smaller base that remains.
Fill in the following:
- Buy price per share
- Current price per share
- Number of shares
Results appear immediately — there is nothing to submit. Changing a field rewrites the link, so you can share the exact scenario you are looking at.
The order the fields are filled in makes no difference to the result — the calculator recomputes the whole formula from whatever is currently in every field, not step by step. That means it is safe to adjust one number, watch the result change, and adjust it back, without worrying about resetting anything first.
Why stock loss matters
Most people who look up a stock loss calculation already have a specific number in mind — a quote, an offer, a target — and want to check it rather than learn the theory behind it. This page is built for that: enter your own figures, see the result immediately, and change any field to see how the answer moves without redoing the arithmetic from scratch each time.
It is also useful as a sense check before signing anything. A quote, an offer letter or a spreadsheet from someone else can contain an error, an optimistic assumption, or simply a different convention for rounding — running the same inputs through an independent calculator is a quick way to confirm a number before relying on it.
The reason a page like this exists at all, rather than leaving the calculation to a spreadsheet or a textbook appendix, is that the formula behind stock loss is fiddly enough to get wrong by hand but not complicated enough to need specialist software. That middle ground — real enough maths to matter, simple enough to check instantly — is exactly what a dedicated calculator is for, and it is why the same figure recalculated here should match a careful manual calculation almost exactly.
A calculator like this one is often bookmarked and returned to repeatedly over months rather than used once, particularly for anything tied to an ongoing plan such as a mortgage, a savings goal or an investment being tracked. Because the figures live in the web address rather than only in memory, coming back to the same page with updated numbers is quicker than starting from a blank spreadsheet each time.
Worked example
Here is the calculation with the starting values:
- Buy price per share: 40
- Current price per share: 26
- Number of shares: 300
That gives:
- Loss: 4,200
- Loss: 35 %
- Gain needed to break even: 53.85 %
These figures are only the calculator's own starting values, included so the working is visible rather than hidden inside the tool above. Replace them with your own numbers and the same arithmetic applies — nothing about the method changes, only the inputs feeding it.
Reading the result
Use the recovery figure as a reality check, not as a reason to hold. The market has no memory of your purchase price — the only question that matters is whether you would buy this holding today at this price.
Where this goes wrong. Averaging down to lower the break-even price. It works arithmetically and often fails in practice, because it concentrates more money in the position that is already going wrong.
A useful check on any unfamiliar result is to compare it against a rough mental estimate first — round the inputs to convenient numbers and see whether the calculator's answer lands in roughly the same territory. A wildly different figure usually means one of the fields was entered in the wrong unit, most often a percentage typed as a whole number where a decimal was expected, or the reverse.
In the UK, realised capital losses can be set against capital gains in the same year, and carried forward indefinitely if reported to HMRC in time. The loss has to be crystallised by selling — an unrealised loss does nothing.
Selling to bank a loss and buying the same share back within 30 days causes the disposal to be matched against the repurchase for capital gains purposes, which cancels the loss. It exists specifically to prevent that manoeuvre.
The headline figure is loss. With 40 buy price per share, 26 current price per share and 300 number of shares, that comes to 4,200. Change any field and the figure moves with it.
Whenever one of the underlying figures changes — a new interest rate, a different balance, an updated term — since the result only reflects what is currently in the fields. There is no need to keep a separate record of past results; the web address for a filled-in version already carries the figures used to produce it.
Not unless a tax rate or a fee is explicitly one of the inputs above. Where it is not, the figure shown is a gross calculation, and any tax due depends on your personal circumstances and current tax rules, which are worth checking separately.
The arithmetic itself is exact — the calculator applies the formula shown above precisely, with no rounding until the final figure is displayed. The uncertainty, where it exists, is entirely in the inputs: an estimated rate or an approximate balance carries that same approximation through to the result.