Blog › Sales Tax Calculator guide
How to Calculate Sales Tax Both Ways
Sales tax math has two directions and people constantly need the less obvious one: adding tax to a price is easy, but extracting the tax from a total you already charged is where errors creep in. This guide covers both, with the formulas, and gives you the calculator for any rate.
Step by step
- Open the Sales Tax Calculator and enter the amount.
- Enter your tax rate as a percentage.
- To add tax: read the tax amount and the total with tax.
- To extract tax: use the tax-included mode to see the pre-tax amount and the tax inside your total.
Try it now — free, no sign-up
Open the Sales Tax Calculator tool
Open the Sales Tax Calculator tool
Tips for better results
- Adding tax: Total = Amount × (1 + rate). Extracting tax: Pre-tax = Total ÷ (1 + rate) — dividing by 1.08 at 8% is NOT the same as subtracting 8%.
- Keep the rate current — jurisdictions change rates, and using last year's rate under-collects.
- On invoices, show the tax as its own line; clients and accountants both prefer it visible.
Frequently asked questions
How do I add 8% tax to $100?
Tax = $100 × 0.08 = $8. Total = $108. In general: total = amount × (1 + rate as a decimal).
How do I find the tax inside a $108 total at 8%?
Pre-tax = 108 ÷ 1.08 = $100, so the tax inside is $8. Never just take 8% of 108 (that gives $8.64 — wrong).
Does the calculator work for VAT and GST too?
Yes — the math is identical. Enter your VAT/GST rate and use add or extract mode the same way.