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A cart total settles in a fixed order: goods, then the discount off that, then shipping and tax — so a percentage quoted against 'the order' means nothing until it says against what.
A cart total is built in a fixed order and reading it out of order is where most mistakes start. The subtotal is the goods alone. The discount comes off that. Shipping and tax are then calculated — and whether they are calculated before or after the discount changes the total, which is why a percentage quoted against 'the order' is ambiguous unless it says against what.
Every figure published on this site is the discount as a proportion of the SUBTOTAL, because that is the only base that is the same on every cart. Across 818 reads the rate has held at exactly 10% of subtotal, on baskets from $40.00 to $1,800.00.
A percentage of an odd subtotal rarely lands on a round cent. Stores round, and a cent of difference between your arithmetic and the cart is rounding, not a different rate. A dollar of difference is a different rate.
This page describes how to read the figures a cart shows you. It does not tell you the cart is honest; for that you need the figures dated and kept, which is what the ledger is for.
If a store applies a code before tax and shipping, the discount is computed on the goods alone. If it applies after, the same percentage comes off a larger number and you save more. Stores differ, and almost none say which they do. The only way to know is to read the cart: put the code in, and see whether the discount line is exactly the percentage of the subtotal or of something larger. On the carts behind this site it is always the subtotal — 818 reads, every one of them exactly 10% of the goods.
A cart total is not one number arrived at in one move. Goods come first, then any discount against the goods, then shipping, then tax on whatever the jurisdiction taxes. The order is what makes a percentage meaningful: ten percent off the goods and ten percent off the final total are different amounts, and a page that says only 'ten percent off' has not told you which. When you check a reduction, check what it was taken against, not just what it came to.
Shipping is usually assessed on the post-discount subtotal, which is why a discount can push an order under a free-shipping threshold and cost more than it saved. Tax is generally calculated after both, on the amount actually being charged. Neither is affected by a discount code in the sense people expect: the code does not reduce tax directly, it reduces the base the tax is calculated on. Reading the lines in order tells you exactly where each amount came from, and makes a cart that does not add up immediately visible.
If the lines do not reconcile — subtotal minus discount plus shipping and tax not equalling the total — something is being applied that is not shown. Occasionally that is a rounding convention. More often it is a fee introduced late, or a discount smaller than the line claims. Either way, the useful response is the same: do not proceed on a total you cannot derive from the lines above it. A store that shows its arithmetic is making itself checkable, and one that does not is asking for trust at exactly the point where trust is least necessary, because the numbers are right there.
The claims on this page about reading a cart total line by line can be checked against sources outside this site: