What invoicing arithmetic covers
Getting paid the right amount involves more arithmetic than it first appears. An invoice total needs to account for quantities, a possible discount, and any applicable tax — all in the right order. After you receive payment, a processor fee may reduce the amount that reaches your account. If a payment is late, calculating the interest or fee owed requires a formula based on your contract rate, the invoice amount, and the number of days overdue. The three calculators in this category handle each of these steps.
Invoice calculator: building the total correctly
The Invoice Calculator takes four inputs: quantity, rate per unit, a discount percentage and a tax rate. It builds the total in a specific order that prevents a common arithmetic error.
First, it multiplies quantity by rate to get a subtotal. Then it applies the discount to that subtotal, not to the post-tax figure — discounts should reduce the taxable base, not come off after tax has been added. Tax is applied to the discounted subtotal. The final total is the amount you put on the invoice.
This order matters. Applying a 10% discount to a $1,000 subtotal and then adding 20% tax gives a different result than adding tax first and then taking the discount. The correct order — discount before tax — is what the calculator uses, and the breakdown shows each step so you can verify it matches your contract terms and local tax rules.
The calculator is country-neutral: it does not know whether your "tax rate" is VAT, GST, sales tax, or something else. You enter the rate; the formula applies it. If your jurisdiction has a specific rule about how discounts interact with tax calculations, check that rule before relying on this output.
Payment processor fees: what platforms take
Every time a client pays through an online platform — Stripe, PayPal, Square, a freelance marketplace — a fee comes out before the money reaches your bank account. That fee is typically a percentage of the transaction plus a small fixed amount. The Payment Processor Fee Calculator takes these two components and shows you the fee in dollars, the net amount you receive, and what you'd need to charge if you wanted the client to cover the fee.
The formula is: fee = (amount × percentage) + fixed amount; net = amount − fee. This is the standard structure used by most processors. Some platforms have tiered fees, volume discounts or currency-conversion charges that are not modelled here — for those, the processor's own fee schedule is the authoritative source. This calculator handles the common case.
Knowing the net amount in advance helps you decide whether to absorb the fee, pass it to the client, or build it into your rate. A 2.9% + $0.30 fee on a $500 invoice takes $14.80, leaving $485.20. On a $50 invoice the same structure takes $1.75, but that's a 3.5% effective rate — fees are proportionally larger on smaller transactions.
Late payment fee calculator: putting the contract into numbers
Most freelance contracts specify an interest rate for late payments — something like "1.5% per month" or "18% per annum." The Late Payment Fee Calculator turns that rate into a dollar amount based on how many days an invoice has been overdue.
The formula uses annual rate rather than monthly because it produces a consistent result regardless of which month the payment is late: fee = invoice amount × (annual interest rate ÷ 100) × (days late ÷ 365). At an 18% annual rate on a $2,000 invoice that is 30 days late: $2,000 × 0.18 × (30 ÷ 365) = approximately $29.59 in interest, making the total owed $2,029.59.
Whether you can charge a late fee, and what rate is enforceable, depends on your contract terms and the applicable law in your jurisdiction. This calculator computes the arithmetic from the numbers you enter; it does not determine whether a particular rate is legal or enforceable in your specific situation. Review your contract and consult a legal professional if there is any question about enforceability before adding fees to an outstanding invoice.
How these tools connect
A typical workflow: use the Invoice Calculator to confirm the total before sending. Note the net amount after processing fees so you know what actually lands in your account. If a payment doesn't arrive on time, use the Late Payment Fee Calculator to work out the interest to add to the demand. Each tool handles one step; together they cover the math of sending an invoice and following up on it.
All three calculators return estimates based on what you enter. Confirm each output against your own contract terms, the processor's published fee schedule, and any local rules about invoicing, tax and interest before you act on it.