For one percentage discount, calculate original × (1 - percentage / 100). If the result still looks wrong, check the percentage’s input scale, the raw numeric calculation, the point where your code rounds, and the currency formatting—in that order. Without the inputs and code, the exact bug cannot be identified, but this sequence isolates the usual causes.
1. Check whether the percentage is entered as 20 or 0.20
A percentage entered as 20 means 20 percent; divide it by 100 in the calculation. A value already entered as 0.20 is the decimal rate and should not be divided again. Decide which form your input uses, then convert it exactly once.
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function discountedPrice(original, percentage) {
return original * (1 - percentage / 100);
}
const result = discountedPrice(49.99, 20);Here, result is the numeric price after a single 20% discount. If your interface accepts decimal rates instead, make that contract explicit and use original * (1 - rate).
2. Confirm the formula and discount sequence
For one discount, the equivalent steps are to calculate discount = original * percentage / 100, then subtract it from the original. Compare those intermediate values with your expected result; this can expose a reversed subtraction, a missing division by 100, or a variable containing a different amount than you think.
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If discounts are applied one after another, do not generally add the percentages. For example, applying 20% and then 10% means multiplying by 0.80 and then 0.90; the combined reduction is 28%, not 30%.
3. Inspect the number before formatting
JavaScript Number uses binary floating-point representation, so some decimal fractions cannot be represented precisely. MDN notes that “decimal fractional numbers cannot be represented precisely in floating point.” That approximation can matter when a computed value is close to a rounding boundary, but it should not be assumed to explain every unexpected price. Reproduce the discrepancy before changing the arithmetic strategy.
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Log each stage separately so you can see whether the error appears in input handling, arithmetic, rounding, or display:
const original = 49.99;
const percentage = 20;
const rate = percentage / 100;
const discount = original * rate;
const finalPrice = original - discount;
console.log({ original, percentage, rate, discount, finalPrice });Keep values used in calculations numeric. Avoid formatting an intermediate amount into a string and parsing that display string back into the calculation state.
4. Decide when and how the application rounds
There is no universally correct rounding point for every price calculator. Requirements can depend on the currency, jurisdiction, whether prices are calculated per item or on a total, and the application’s pricing rules. Decide whether to retain precision through intermediate steps and round per item, per calculation step, or only at the final total; then test that policy against the requirements that apply to your product.
toFixed(2) returns a string with two fractional digits. It formats the represented number; it does not make earlier arithmetic exact. Use it only when fixed two-digit output is actually appropriate, and keep its string result separate from the numeric value used for further calculations. See MDN’s Number.prototype.toFixed() reference.
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5. Format currency at the output boundary
For locale-aware currency display, use Intl.NumberFormat with a deliberate locale and ISO currency code. Locale controls conventions such as decimal separators and digit grouping, while the currency affects the fractional digits used. Do not confuse a formatting difference—such as a comma instead of a period—with a different numeric result.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchconst display = new Intl.NumberFormat("en-US", {
style: "currency",
currency: "USD",
}).format(finalPrice);
console.log(display);This example displays the calculation as US-dollar currency for the en-US locale; it does not establish a universal rounding policy for stored or transactional values. See MDN’s guides to JavaScript internationalization and the Intl.NumberFormat() constructor. If you use explicit fraction-digit or newer rounding options, check that the browsers or runtime versions you target support them.
6. Reproduce the mismatch with a boundary case
Test a price and percentage whose exact result is close to a currency rounding boundary. Compare the raw result, any value rounded by your own code, and the final formatted text. If those values differ, you can identify which stage creates the mismatch rather than guessing that floating point is responsible.
- Record the original amount and its type.
- Record the entered percentage and whether it is a whole-number percentage or decimal rate.
- Record the discount amount and unformatted final number.
- Record any rounding operation and the formatted output.
- Repeat using the same locale, currency, and calculation path as the failing case.
What to include when asking for a specific diagnosis
To pinpoint a bug, provide the original amount, the entered discount, the expected price, the actual price, the currency, and the code path that performs calculation and rounding. Without those details, it is possible to identify common failure points but not determine which one is responsible in a particular calculator.
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Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
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