Sales Tax Calculator: Forward and Reverse Tax Math
Add sales tax to a net price or extract the tax already inside a gross amount. Shows the formula, the rounding rule, and the reverse-tax math step by step. Private — runs entirely in your browser.
Sales Tax Calculator workspace
Forward & Reverse
Add tax to a net price for invoicing, or work backwards from a gross receipt to see the net and the tax hidden inside it.
Stated Rounding Rule
Every figure is rounded half-up to cents, and the rule is printed next to the result — no mystery pennies.
Formulas On Display
gross = net × (1 + r) forward; tax = gross × r/(1 + r) in reverse. The substituted math is shown under every answer.
100% Client-Side
Prices and rates are computed locally in your browser. Nothing you type is uploaded or attached to page analytics.
Sales tax calculator: add tax forward, extract it in reverse
A sales tax calculator answers two opposite questions with the same rate. Forward: given a net price and a rate, what is the tax and the final total? Reverse: given a tax-inclusive gross, how much tax is hiding inside it? This tool shows the formula, substitutes your numbers, states its half-up rounding rule, and runs entirely in your browser — private, free, currency-neutral.
How to use the sales tax calculator
- Pick a direction: Add tax (net → gross) for pricing and checkout, or Remove tax (gross → net) for receipts filed tax-inclusive.
- Type the price you have — the net price before tax in forward mode, or the gross total in reverse mode.
- Enter your tax rate as a percent; decimals are welcome (8.25 or 8.25% both parse).
- Read the large result, the tax line, and the formula with your numbers substituted underneath it.
- Press Copy summary to capture a one-line record like “$80 + 8.25% ($6.60) = $86.60” for your notes.
- Check the rounding note: every figure is rounded half-up to cents, matching paper arithmetic.
The forward formula: gross = net × (1 + r)
Tax is a percent of the purchase price, and the total is price plus tax. OpenStax's free Prealgebra text states the model directly in its sales tax applications chapter: sales tax = tax rate × purchase price, then total cost = purchase price + sales tax. Writing r as the rate as a decimal (8.25% → 0.0825) collapses the two steps into gross = net × (1 + r).
For the official side of the ledger, the IRS discusses deductible state and local general sales taxes in Topic no. 503, and GOV.UK's guide on charging and reclaiming VAT demonstrates both directions arithmetically: multiply the ex-VAT price by 1.2 to charge, divide the VAT-inclusive price by 1.2 to reclaim — the same forward and reverse math this calculator performs at any rate you enter.
The four formulas, side by side
| Quantity | Formula | Worked example (r = 8%) |
|---|---|---|
| Gross from net | gross = net × (1 + r) | 80 × 1.08 = 86.60 |
| Tax amount | tax = net × r | 80 × 0.08 = 6.40 |
| Net from gross | net = gross ÷ (1 + r) | 108 ÷ 1.08 = 100.00 |
| Tax inside a gross | tax = gross × r/(1 + r) | 108 × 0.08/1.08 = 8.00 |
Worked examples: input → output
Forward · $80 at 8.25%
tax = 80 × 0.0825 = $6.60 · gross = 80 + 6.60 = $86.60
Reverse · $108 gross at 8%
net = 108 ÷ 1.08 = $100.00 · tax inside = 108 − 100 = $8.00
Rounding · $9.50 at 7%
9.50 × 0.07 = 0.665 exactly — half-up rounds the cent up to $0.67, giving a $10.17 total.
Edge case · the classic reverse-tax mistake
Given a $108 gross at 8%, subtracting “8%” directly computes 108 × 0.08 = $8.64 and calls the rest “net” ($99.36). Both figures are wrong: the true tax is $8.00 because the 8% applied to the $100 net, not to the $108 total. Always divide by (1 + r); never subtract the bare percent.
No built-in rate tables — deliberately
Many tax calculators ship a lookup table of state or national rates. Tables rot: rates change mid-year, local districts stack on top of state rates, and some categories carry reduced rates while others are exempt. A stale table produces confidently wrong totals. This tool takes the rate as an input instead, so the output is never more stale than the number you just typed — and the formulas above stay valid for VAT, GST, or any percentage-based levy, in any currency.
The rounding choice matters more than it looks. Half-up-to-cents matches how most people (and many invoice systems) round on paper, but binary floating-point arithmetic makes exact .5-cent cases slippery — 0.665 stored internally is 0.6649999…. The calculator rounds against the recovered decimal string, so the $9.50-at-7% example lands on $0.67 the way your handwritten receipt says it should.
Estimates for understanding — not tax advice
Results are arithmetic estimates meant for shopping decisions, invoice checking, and understanding what a receipt contains. They are not tax advice. Real rates vary by jurisdiction, product category, and customer status, and they change over time — confirm the applicable rate with your tax authority (such as the IRS or HMRC) before filing, invoicing, or remitting anything.
Runs 100% in your browser
Your prices and rates never leave your device. Forward math, reverse extraction, rounding, and the summary copy are all computed locally in JavaScript — no uploads, nothing leaves your device. Toolk's page-view analytics count visits but receive none of the amounts you enter. I hand-checked both modes before shipping: $80 at 8.25% forward returns $6.60 tax / $86.60 gross, $108 at 8% reverse returns $8.00 tax / $100.00 net, and the $9.50-at-7% case confirms the half-up cent rounding.
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Last updated: August 23, 2026 · Runs 100% in your browser — no uploads, nothing leaves your device.
Frequently asked questions
Why does the reverse mode divide instead of subtracting the rate?
Because the gross already contains the tax, the rate applies to the smaller net figure — not to the gross. Dividing by 1 + r undoes the multiplication that created the gross ($108 ÷ 1.08 = $100). Subtracting 8% of $108 gives $99.36 and a tax of $8.64, which quietly overstates both. The UK's own VAT guidance uses exactly this divide-by-1.2 method for extracting VAT.
What rounding rule does this sales tax calculator use?
Half-up rounding to two decimal places: a third decimal of 5 or more rounds the cent up ($0.665 becomes $0.67), anything less rounds down. The rule is stated beside every result so invoiced amounts reconcile with hand calculations, rather than depending on whatever default your accounting software picked.
Does the calculator assume dollars, or can I use any currency?
Any currency. Inputs are plain numbers with two-decimal outputs, so enter figures in euros, pounds, yen, or dollars alike — no currency symbol is assumed and none is added. The math is identical; only your jurisdiction's rate changes, which is why you type it yourself.
Why doesn't the tool include my state or country's tax rate automatically?
Rates change constantly and stack differently by city, county, and product category — a built-in table would go stale and silently produce wrong invoices. This calculator deliberately takes the rate as input so the result is always exactly as current as the number you enter. Check your tax authority's published rate before filing or invoicing.
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