FairSpend explains its math so you can check a result yourself. These tools operate on numbers you enter; they do not retrieve live store prices or decide what a group should consider fair.
Price and quantity
For an eligible offer, the calculator starts with the entered price, applies a percentage discount, subtracts a coupon or credit within the price model, applies the entered sales tax rate, and includes entered shipping or fees. It then divides the modeled checkout total by a compatible package quantity. Weight, volume and item counts are separate measurement families. The two offers must describe comparable products; a smaller unit price does not account for quality, shelf life or purchase minimums.
Display values may be rounded for readability. When a comparison is close, check the seller’s actual checkout and the unrounded relationship implied by the original price and quantity. Our guides show intermediate values where they help explain a result.
Discounts and target price
Sequential discounts multiply: a first discount of 20% and a second of 10% leave 0.8 × 0.9 = 0.72 of the original price, a 28% overall reduction before any tax. A target unit price is a personal threshold: total price ÷ usable units is compared with your entered goal. Neither tool knows whether a coupon stacks, expires or applies to shipping.
Shared expenses
For an equal group split, the tool totals entered expenses and divides by the number of participants. It compares each person’s payments with that share and suggests transfers between people with positive and negative balances. The restaurant quick tool uses the entered bill, tip percentage and diner count. The roommate quick tool allocates a bill in proportion to nonnegative entered weights. None of these tools executes a payment or chooses who should participate in an expense.
What we check when publishing guides
FairSpend writes original explanations with a reproducible numeric example, states the formula and identifies material limitations. We distinguish example prices from live offers. When an external conversion or consumer guidance is relevant, we link to a primary source such as NIST measurement equivalencies or FTC shopping guidance. We review corrections when a reader identifies a reproducible error.
Corrections and feedback
Send the page link, input numbers, expected result and a brief explanation through Contact FairSpend. We check the example or calculation and revise the affected page when the report is confirmed. Please do not send bank details, card numbers or passwords. These calculations are general information and are not personal financial, tax or legal advice. Last reviewed: September 22, 2026.