A price with a 30% discount, then 14% tax added on top — so what do you actually pay? These sums look simple until they chain together, and that is when mistakes creep in. On an invoice, a mistake means a wrong amount.
This guide shows how to work out percentages accurately inside your browser, and where people usually go wrong.
The four basic calculations
Almost everything you need falls into four cases:
| Case | Example | Result |
|---|---|---|
| A percentage of an amount | 15% of 2,400 | 360 |
| An amount plus a percentage | 2,400 + 15% | 2,760 |
| An amount minus a percentage | 2,400 − 15% | 2,040 |
| What percentage one amount is of another | 360 of 2,400 | 15% |
Recognising which case you have is half the job. The other half is applying the operations in the right order.
The biggest mistake: the order of discount and tax
Take 1000, a 20% discount and 14% tax.
- Discount then tax: 1000 → 800 → 800 + 14% = 912
- Tax then discount: 1000 → 1140 → 1140 − 20% = 912
Here both give the same answer. But if you combine the rates first (20% + 14% = 34%) you get a completely different figure. The rule: apply each rate separately, in the order the invoice specifies.
How to calculate in your browser — step by step
Step 1: open the tool
Go to TaskDo's percentage calculator.
Step 2: choose the calculation type
Pick the case you need from the four above.
Step 3: enter the numbers
Type the amount and the rate. The result appears immediately.
Step 4: verify with the inverse
To check: divide the result by the original and work out the rate — you should get back what you entered.
Local processing vs sites that want your numbers
| Criterion | In your browser (TaskDo) | A server-side site |
|---|---|---|
| Where the maths happens | On your device | On a third party's server |
| The amounts you enter | Never transmitted | Usually logged |
| Your prices and margins | Stay private | Become someone else's commercial data |
| Works after page load | Works offline | Does not work |
Selling prices and profit margins are among the most sensitive numbers in any business.
Common mistakes
- Adding rates before applying them: 20% off then 14% on is not a net 6%.
- Confusing "a percentage of" with "plus a percentage": "15% of 2400" is not "2400 plus 15%".
- Discounting from the reduced price: a discount applies to the original price, not to an already-reduced one.
- Rounding too early: on invoices, round at the end, not at every step, or errors accumulate.
- Skipping the inverse check: one step catches most mistakes.
Practical tips
- Separate every rate: write out the invoice steps, then apply them in order.
- Always check in reverse: from the result back to the original.
- Round only at the end: avoid accumulating small differences.
- Note your fixed rates: your tax rate and usual margin.
Frequently asked questions
Are the numbers I enter sent anywhere?
No. The calculation runs entirely in your browser and no value is transmitted to anyone.
Does the tool handle discounts and tax?
Yes, it covers the four basic cases: a percentage of an amount, adding a rate, subtracting a rate, and what percentage one amount is of another.
In what order should discount and tax be applied?
Apply each rate separately, in the order the invoice specifies. Combining the rates first gives a different answer.
Is it accurate for large amounts?
Yes, the arithmetic is exact — just keep rounding to the final step.
Is there a limit on calculations?
No, use it as much as you like.
Does it work on a phone?
Yes, on any device with a modern web browser.
Related tools
- Number to words — spell the amount out for the invoice
- Unit converter — length, weight, area
- Colour converter — convert colours and check contrast
- Age calculator — ages and durations
Summary
The biggest invoice mistake is combining rates before applying them. Apply each separately, and always check in reverse. And the key idea: your prices and margins stay with you.