Reverse Percentage Calculator (Price Before Tax or Discount)
Updated · Free · No sign-up · Runs in your browser
Work backwards from a final amount: the price before sales tax was added, the original price before a discount, or the whole when you only know a part and its percentage.
The tool divides instead of subtracting — the mistake that gives the wrong answer on most quick calculations.
Original amount
100.00
Original 100.00 + 8% (8.00) = 108.00 · 108.00 ÷ 1.08
Common mistake: subtracting 8% from the total gives the wrong answer (108 − 8% = 99.36, not 100). Divide by 1.08 instead.
Price before tax for a $100.00 total
| Sales tax | Before tax | Tax |
|---|---|---|
| 5% | $95.24 | $4.76 |
| 6% | $94.34 | $5.66 |
| 7% | $93.46 | $6.54 |
| 8% | $92.59 | $7.41 |
| 8.25% | $92.38 | $7.62 |
| 9% | $91.74 | $8.26 |
| 10% | $90.91 | $9.09 |
Worked examples
$108 including 8% tax
- 108 ÷ 1.08
→ $100.00
$75 after 25% off
- 75 ÷ 0.75
→ $100.00
$53.99 including 8.25% tax
- 53.99 ÷ 1.0825
→ $49.88 (tax $4.11)
15 is 20% of what?
- 15 × 100 ÷ 20
→ 75
Reverse percentage formula
- After an increase of P%: original = final ÷ (1 + P/100).
- After a decrease of P%: original = final ÷ (1 − P/100).
- When X is P% of a whole: whole = X × 100 ÷ P.
How to find the original price before a discount
- Choose "Sale price → original price".
- Enter the price you paid and the discount percent.
- The original price is the sale price ÷ (1 − discount). $75 at 25% off was $100.
Why you cannot just subtract the percentage
8% of $108 is $8.64, but the tax was 8% of the original $100, which is $8. Subtracting from the total uses the wrong base; dividing by 1.08 gives the right one.
Frequently asked questions
How do I calculate the price before tax?
Divide the total by 1 plus the tax rate as a decimal. With 8% tax, $108 ÷ 1.08 = $100.
How do I find the original price before a discount?
Divide the sale price by 1 minus the discount as a decimal. $75 after 25% off: 75 ÷ 0.75 = $100.
Why is subtracting 8% from the total wrong?
Because the 8% was calculated on the smaller original price, not on the total. Subtracting 8% of $108 leaves $99.36 instead of $100.
How do I find the whole from a part and a percent?
Multiply the part by 100 and divide by the percent: 15 is 20% of 15 × 100 ÷ 20 = 75.
Does it work for tips and markups?
Yes. Use "Price with tax → before tax" for any amount that had a percentage added, such as a tip, service charge or markup.
Are my numbers saved?
No. Everything is calculated in your browser.