I've been writing about math for 5 years, and I'm convinced there's only one good way to calculate a tip

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• The best way to calculate a tip is simple: multiply the sales tax.
• After writing about math for five years, I'm surprised there's still so much variation in how people choose to do this everyday math.
• Most jurisdictions charge a sales tax, which can take a lot of the work out of estimating 15%, 20%, or 25% of a bill.

One of the perennial challenges to eating at a restaurant or ordering delivery is figuring out how much to tip.

Fortunately, in most cities and states, there's already an entry on your receipt that makes estimating a tip much easier: sales tax.

According to the Tax Foundation, 46 states have a state sales tax, local sales taxes, or both, and combined state and local taxes usually range between 5% and 10%. If you live in such a jurisdiction, you can use the already calculated sales tax to come up with a good estimate for what you should pay for a tip.

Read more: How much to tip in every situation, from Uber drivers to your hairstylist

I live in New York City, where sales tax is 8.875%. That's about 9%.

So when I settle the bill at a restaurant, I either multiply by 2 to leave a bit below 18%, or, if I'm leaving a larger tip (which I usually do, because service work is unforgiving) I multiply by 3 to leave between 26% and 27%.

This is all very easy for me because I know my local tax rate, and it was very much worth the 60 seconds it took to find out. You can find your own through this calculator, or get a decent idea of what sales tax rates look like in your state at the Tax Foundation's website.

Exact sales tax rates vary widely across different states, counties, and cities. But since most jurisdictions with a sales tax have rates that fall in that 5% to 10% range, simply multiplying sales tax by 3 will get you a respectable 15% to 30% tip.

If you can stomach a little extra math, here are some examples of how sales tax can help you estimate a roughly 15%, 20%, or 25% tip, based on the approximate tax rate in your state and city. It's all basic multiplication, so there shouldn't be any surprises:

• 5% sales tax: Since the most common tipping brackets are multiples of 5%, this one is straightforward. Multiply your sales tax by 3 to get a 15% tip, by 4 to get a 20% tip, and by 5 to get a 25% tip.
• 6% sales tax: Multiply by 3 to get to 18% and adjust slightly up or down if you so desire, and by 4 to get to 24%.
• 7% sales tax: Doubling sales tax will get to 14%, and adjusting upward will put you at 15%. Multiplying by 3 gets to 21%, which can be rounded down for a 20% tip or bumped up to get to around 25%.
• 8% sales tax: Multiplying by 2 gives 16%, which is right around the 15% tip range, and multiplying by 3 goes to 24%, near the 25% range.
• 9% sales tax: Doubling the sales tax here gets to 18%, and multiplying by 3 gets to 27%.
• 10% sales tax: Multiplying a 10% tax by 2 gets us right to 20%. To estimate a 15% tip, take half of the sales tax and add it back to the original tax. For a 25% tip, take half the sales tax and add it to twice the tax.