You’re standing in a shop holding a jacket. The tag says $80, a sign says “30% off today”, and you know tax will be added at the till. Is it worth it? You could reach for your phone, but there’s a faster way.
Once you know a few tricks, you can calculate a discount and sales tax in your head in about ten seconds. This guide explains how, with real examples for shoppers in the US, UK, EU, Australia and New Zealand.
First, Know How Tax Works Where You Live
Before any maths, check one thing: is tax already in the price?
- United States: Sales tax is added at checkout. It varies by state and sometimes by city, from 0% in places like Oregon to roughly 7-10% elsewhere.
- United Kingdom: VAT is already included in the shelf price. The standard rate is 20%.
- European Union: VAT is included too. Standard rates differ by country, for example 19% in Germany, 20% in France and 23% in Ireland.
- Australia: GST is 10% and included in the displayed price.
- New Zealand: GST is 15% and also included.
So in the UK, EU, Australia and New Zealand, the price on the tag is usually the price you pay, and you only need to work out the discount. In the US, you do both steps.
The Order Doesn’t Matter (Good News)
Many people worry about whether the discount comes before or after tax. Mathematically, it makes no difference. Taking 30% off and adding 8% tax gives the same total as adding the tax first and then taking 30% off, because you’re only multiplying two numbers together.
In practice, US shops apply the discount first and charge tax on the lower price, so that’s the order we’ll use. It’s also easier to do in your head.
Step 1: Work Out the Discount
The secret is to anchor on 10%. Moving the decimal point one place to the left gives you 10% of any price.
- $80 → 10% is $8
- £45 → 10% is £4.50
- €120 → 10% is €12
From 10%, you can build almost any discount:
| Discount | Quick method |
|---|---|
| 10% | Move the decimal one place |
| 20% | Find 10%, then double it |
| 25% | Halve the price, then halve again |
| 30% | Find 10%, then triple it |
| 50% | Halve the price |
| 15% | Find 10%, then add half of that |
Example: A $60 shirt is 20% off. 10% is $6, double is $12, so the discount is $12 and the new price is $48.
Step 2: Add the Sales Tax (US Shoppers)
Use the same 10% anchor, then adjust.
- Tax of 5%: Find 10% and halve it.
- Tax of 8%: Find 10% and subtract 2% (a fifth of the 10% figure).
- Tax of 7%: Find 10% and subtract 3%.
- Tax of 10%: Just use the 10% figure.
A Full Worked Example
Let’s go back to that $80 jacket with 30% off and 8% sales tax.
- Discount: 10% of $80 is $8. Triple it to get $24.
- Sale price: $80 − $24 = $56.
- Tax: 10% of $56 is $5.60. For 8%, take off 2% (about $1.12), which gives roughly $4.48.
- Total: $56 + $4.48 = about $60.48.
If you want a quick estimate, round $4.48 to $4.50 and say “about $60 and a half.” Your phone will agree within a few cents.
The Stacked Discount Trap
Shops love to say “30% off, plus an extra 10% at the till.” It sounds like 40% off. It isn’t.
The second discount applies to the already reduced price. So the real saving is:
- Take 30% off first, so you pay 70% of the price.
- Take 10% off that, so you pay 90% of 70%, which is 63%.
- Your total saving is 37%, not 40%.
A handy rule: two stacked discounts are always slightly smaller than their sum. If you see 20% plus 10%, the real figure is 28%, not 30%.
Finding the Tax Already Inside a Price (UK, EU, AU, NZ)
Sometimes you want to know how much VAT or GST is hidden in a price, for example when you’re comparing prices across borders or checking a business receipt. Instead of dividing by 1.2 or 1.15, use these fractions of the final price:
- UK VAT (20%): The tax is one sixth of the price. On £60, the VAT is £10.
- Australia GST (10%): The tax is one eleventh of the price. On A$55, the GST is A$5.
- New Zealand GST (15%): The tax is three twenty-thirds of the price. On NZ$115, the GST is NZ$15, because 115 ÷ 23 = 5, and 5 × 3 = 15.
- Germany VAT (19%): Divide the price by 6 as a rough guide. It’s a bit high, but close enough for a quick check.
To remove the tax and see the pre-tax price, just subtract that portion.
Rounding Like a Pro
You don’t need perfect accuracy in a shop. You need a number that’s close enough to decide.
- Round the price to something friendly. Treat $79.99 as $80.
- Round the tax up slightly so you never get a nasty surprise at the till.
- Estimate the total first, then check the receipt. After a few tries, you’ll be surprised how close you get.
Common Mistakes to Avoid
- Adding the discount and the tax percentages together. “30% off and 8% tax” is not “22% off.” Always apply them one at a time.
- Forgetting that tax in the US varies by location. Check the local rate if you’re travelling between states. Some items, such as groceries or clothing, may be taxed differently.
- Assuming a sale price includes tax in the US. It usually doesn’t, unlike in the UK, EU, Australia and New Zealand.
- Trusting “up to 70% off” signs. The headline number usually applies to one or two items. Always check the tag on the item you’re holding.
Quick Practice Round
Try these in your head, then check the answers below.
- A £90 coat is 20% off. What’s the sale price?
- A A$120 pair of shoes is 25% off. What do you pay?
- A $50 bag is 40% off with 10% sales tax. What’s the total?
Answers: 1) £72. 2) A$90. 3) $30 sale price, plus $3 tax, so $33.
Frequently Asked Questions
Do I calculate the discount before or after tax?
The final total is the same either way. Most shops take the discount first and then add tax, so that’s the easiest way to do it in your head.
Is VAT included in the price in the UK and EU?
Yes. Consumer prices in shops and online normally include VAT, so the tag price is what you pay.
Does GST get added at the till in Australia and New Zealand?
No. GST is normally included in the displayed price in both countries.
What’s the fastest way to work out 15% off?
Find 10%, then add half of that. For $40, 10% is $4 and half is $2, so 15% is $6.
Final Thoughts
Mental maths for shopping comes down to one habit: find 10% first, then build from there. With that one move, you can handle most discounts and tax rates without a calculator. Practise on your next shopping trip, and you’ll soon be the person who knows the real price before the cashier says it.
If you want to double-check bigger purchases, a simple percentage or discount calculator is a handy backup, but for everyday shopping, your head is faster.
This article is for general educational purposes only. Tax rates change, so check your local government’s website for current rates.