If you ever have a basket full of shopping goods and want to get a rough idea of the sales tax you’ll be paying, here’s a guide for how to quickly and accurately estimate your tax bill.

This depends on the state you are in – some states have rates as low as 0% (no sales tax), all the way up to nearly 10%. You can determine your state’s rate by checking a receipt on a recent purchase. The total sales tax will be displayed and usually falls between 5 and 9%.

(1) Round your purchase to the nearest ten. If your purchase total is $476.99, round it to $480.

(2) Know your state tax rate, and round it to the nearest whole number. This can be as low as 5%, or as high as 9%.

(3) Follow these tricks for estimating your tax and total:

5%:

(a) Divide your purchase total by 10, ie: delete the last zero in the total. So $480 becomes $48.

(b) Now divide by 2. So $48 becomes $24, your approximate sales tax.

6%:

Follow the steps for the 5% tax. Now, divide your original amount by 100 (shift the decimal place over by two), so $480 becomes $4.80, or about $5.

Now add this amount to the 5% sales tax: $24+$5=$29.

7%:

Same as 6%, but add the total divided by 100 two times. So $24+$5+$5=$34.

9%: Divide your total by 10, so $48. Now divide this by 10 again, and subtract: $4.8, or about $5, so $48-$5=$43.

8%: Follow the same steps as for the 9% sales tax, but subtract twice: $48-$5-$5=$38.

If you follow these steps, you’ll be able to quickly estimate the tax on any item you’re purchasing.

Here is another common mental math question: how can you quickly estimate, in your head, a fair tip for your waiter or waitress?

Most advice says we should give 15-20% tip. Here’s how to quickly estimate the top you give to a server.

Let’s say your restaurant bill is $53.

(1) Divide this by 10: $5.30

(2) Multiply this by 2: $10.60, or about $11.00.

(3) The halfway point between these 2 is the 15% tip, so in this case, about $8.00.

(4) The amount in (2) is a more generous 20% tip.

Pick an amount between (2) and (3) if your service was extra good.