Why does the discounted price in my store's cart differ from the calculator by one cent?

An example that shows the whole gap

A product at 33.35 with 15% off, and a customer buying three. There are two sensible ways to compute the order. Both are correct, and they do not give the same number.

Tracking down the cause yourself

1. Enter the unit price and the rate in the Discount calculator. It shows prices rounded to two decimals, so for the exact figure before rounding, multiply the price by what remains after the discount, such as 33.35 × 0.85.
2. Multiply the displayed unit price by the quantity. If that matches the cart, the store rounds per unit.
3. Enter the cart total before discount with the same rate. If that matches, the store discounts the total.
4. If neither matches, tax is probably entering the sum before or after the discount.

When tax is involved

A percentage discount gives the same result mathematically whether it comes before or after tax, but the rounding does not. A store that rounds the discounted price, then adds tax and rounds again, can land a cent away from one that computes everything and rounds once. To add or remove tax at the rate you enter, use the VAT calculator.

Once you know the cause

• The amount the customer pays is the checkout figure. Quote that one in ads and messages.
• If you are comparing the calculator with a supplier invoice, ask how they round instead of assuming an error.
• The gap does not grow much: at most half a cent per rounding, but it shows up on orders with quantities.

Within the calculator itself, the displayed saving is the original price minus the rounded final price, so the two displayed figures always add back to the original. That is why one unit shows a saving of 5.00 even though the exact discount is 5.0025.