Payment processing is usually the largest line item a small business never negotiates. At 2.9% + $0.30, a business doing $30,000 a month in card volume hands over roughly $10,500 a year — more than most founders spend on software. The calculators here exist so that number stops being a surprise on a monthly statement.
Which one you want
Start with the Fee Comparison calculator if you are choosing a processor or wondering whether to switch: it puts Stripe, PayPal and all three Shopify Payments plans side by side for a single sale amount and shows the exact net payout for each. Use the individual Stripe, PayPal and Shopify calculators when you already know your processor and need a precise figure — each one handles the details the comparison view flattens, including international cards, negotiated rates, and grossing a charge up so you receive an exact amount after fees.
The mistakes these catch
Two errors turn up constantly. The first is comparing headline percentages while ignoring the fixed fee: at a $5 sale, Stripe's $0.30 is 6% on its own, which is why micro-transactions destroy margins that look fine on a spreadsheet. The second is forgetting cross-border and currency-conversion surcharges — an extra 1.5% and 1% respectively on Stripe — which quietly make an international customer meaningfully less profitable than a domestic one at the same price.