Calculator

A calculator built for market maths

This is a plain four-function calculator placed inside a market dashboard for a simple reason: most of the arithmetic that matters when you are watching prices is short. Converting a position size, working out what a percentage move is worth, splitting an order across two exchanges, or checking whether a spread survives a fee — none of these need a spreadsheet, but all of them need to be right.

It runs entirely in your browser. Nothing you type is sent to a server, stored, or logged, so you can use it for personal figures without leaving a trace on this site.

Percentage changes: the calculation people get wrong most often

To find the percentage change between an old price and a new one, subtract the old from the new, divide by the old, then multiply by one hundred. If a stock moves from 240 to 258, that is 18 divided by 240, which is 0.075, or a 7.5 per cent gain. Dividing by the new price instead of the old one is the single most common error, and it understates gains while overstating losses.

The asymmetry of percentages is worth internalising. A 50 per cent loss requires a 100 per cent gain to recover, because you are now calculating from a smaller base. A holding that falls from 200 to 100 must double to get back to where it started.

Working out what a move is worth to you

Multiply your quantity by the price change per unit. Holding 12 shares through a 3.40 move gives 40.80 before costs. For fractional crypto positions the same logic applies: 0.35 of a coin through a 1,200 move is 420. Convert into your home currency afterwards using the live rate on the forex page rather than a remembered one, because a one per cent currency move on a large position is not a rounding error.

When comparing two possible trades, compare the percentage returns rather than the absolute amounts. Absolute profit depends only on how much you committed; percentage return tells you which decision was actually better.

Fees, spreads and the number that actually lands in your account

Trading costs are charged on both sides of a round trip. If an exchange takes 0.10 per cent as a taker fee, a buy and a later sell cost roughly 0.20 per cent in total, so a spread of 0.15 per cent is a loss rather than a profit. Add the bid-ask spread itself, any withdrawal or network fee, and — for cross-border transfers — the currency conversion margin, which is frequently the largest hidden cost of all.

A useful habit is to calculate the break-even move before you commit: total round-trip costs as a percentage of the position tells you how far the price must travel before you are even. Our arbitrage calculator applies exactly this logic automatically for cross-exchange trades.

Compounding and annualised returns

Growth applied repeatedly compounds. An amount growing at a steady rate over several periods is the starting amount multiplied by one plus the rate, raised to the number of periods. Ten thousand growing at six per cent for five years becomes 10,000 times 1.06 to the fifth power, about 13,382. The extra 382 over simple interest is the compounding itself.

The rule of 72 gives a fast mental estimate: divide 72 by the annual percentage rate to approximate how many years a sum takes to double. At six per cent that is roughly twelve years, at nine per cent about eight. It is an approximation, but it is accurate enough for quick comparisons, and it works equally well for estimating how quickly inflation halves purchasing power.

What this tool does not do

This calculator performs arithmetic only. It does not forecast prices, assess risk, account for tax, or tell you whether a trade is sensible. Nothing on this page is financial advice, and USD Euro 360 is not a broker, adviser or exchange. Figures you derive here should be checked against your own broker or exchange statements before you act on them.

Frequently Asked Questions

Sources & further reading

• Investor.gov — compound interest explained
• ECB — euro foreign exchange reference rates