Reverse Calculators
Reverse Percentage Calculator
If 45 is 30% of a number, what was the number? Also reverse a percentage increase or decrease.
Working a percentage backwards
Forward percentages are easy; reversing them is where people lose a receipt discount or a VAT-inclusive price. This is the same family of maths as reverse margin, but for any percentage of a whole. Enter the part and the percentage to find the whole, or the whole and percentage to find the part — depending on the mode you need. Related: reverse discount and reverse margin. VAT net = gross ÷ (1 + rate) is the everyday UK case. Percentages over 100% can appear in growth or markup contexts — the algebra still works if you interpret the business meaning carefully. Grant claims and partnership deals often state a percentage of costs recovered — reverse to see the implied full project spend. Tip calculations in reverse help when a card machine shows only the total charged. Keep a habit of writing the formula beside the number so teammates can audit the step.
Worked example
15 is 20% of a whole → whole = 15 ÷ 0.20 = 75. A VAT-inclusive price of £120 at 20% VAT has net 120 ÷ 1.20 = £100. If someone says a fee is ‘12% of turnover’ and the fee invoice is £3,600, implied turnover is 3600 ÷ 0.12 = £30,000.
Limits and assumptions
Assumes a single clean percentage relationship. Tiered tax bands and compound growth need more steps. Not tax advice — confirm VAT rates for your goods.
Frequently asked questions
How do I find the original from a percentage part?
Divide the part by the percentage as a decimal. If 15 is 20% of a whole, whole = 15 ÷ 0.20 = 75.
Is VAT reversal the same idea?
Yes — net = gross ÷ (1 + VAT rate). Always confirm the VAT rate that applies to the goods.
Can percentages over 100% make sense?
Sometimes for markups or growth. The algebra still works; interpret the business meaning carefully.
How do I find X% of a number?
Multiply the whole by the percentage as a decimal. Reverse mode is for when you know the part instead.
What about successive percentages?
Use discount stacking for sequential discounts rather than adding percentages.