# Percentage Calculator: % Of, % Change, Discount and Reverse

Free percentage calculator: X% of Y, what percent one number is of another, percentage increase or decrease, discount prices with 9% GST, and reverse percentages.

Interactive tool: https://lookup.sg/tools/percentage-calculator

## How this works
A percentage is a number out of 100. X% of Y is X ÷ 100 × Y, so 15% of 280 is 42. To find what percentage A is of B, divide A by B and multiply by 100: 45 out of 60 is 75%.

Percentage change compares the difference with the starting value: (new − old) ÷ old × 100. A pay rise from S$2,400 to S$2,650 is (250 ÷ 2,400) × 100 = 10.42%. Going back from S$2,650 to S$2,400 is a 9.43% fall, because the starting value is now larger.

A discount multiplies the price by (1 − discount). Stacked discounts multiply too, so 30% off then an extra 10% off is 37% off in total, not 40%. GST-registered businesses in Singapore must display prices with GST included; when a price is quoted before GST, the 9% is added after the discount.

A reverse percentage finds the original from the result: divide by (1 + rate) after an increase or (1 − rate) after a decrease. S$109 including 9% GST was S$100 before GST; a S$56 item after 20% off was S$70 before the sale.

## FAQs
### How do I calculate percentage increase?
Subtract the old value from the new one, divide by the old value and multiply by 100. From S$2,400 to S$2,650 is (2,650 − 2,400) ÷ 2,400 × 100 = 10.42%.

### How do I work out 30% off a price?
Multiply the price by 0.70. A S$129 item at 30% off is S$90.30, a saving of S$38.70. If the price is before GST, add 9% afterwards: S$98.43.

### Is 30% off plus an extra 10% off the same as 40% off?
No. The extra 10% applies to the already-reduced price, so the total is 37% off. On S$129 that is S$81.27, not S$77.40.

### How do I find the original price before a percentage was added?
Divide by 1 plus the percentage. A S$109 price including 9% GST was S$109 ÷ 1.09 = S$100 before GST. After a 20% discount, divide by 0.80 instead: S$56 was S$70.

### What percentage is 45 out of 60?
75%. Divide 45 by 60 to get 0.75, then multiply by 100.

## Sources
- [IRAS: Displaying and quoting prices (GST-inclusive prices)](https://www.iras.gov.sg/taxes/goods-services-tax-(gst)/basics-of-gst/invoicing-price-display-and-record-keeping/displaying-and-quoting-prices)
- [IRAS: Current GST rates](https://www.iras.gov.sg/taxes/goods-services-tax-(gst)/basics-of-gst/current-gst-rates)

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Source: Lookup (https://lookup.sg) · Data as of October 2026 · Listings compiled from Singapore government open data (data.gov.sg), OneMap and Overture Maps places. Free listings never show contact details; contact details appear only on Lookup Verified listings.