In the US, doubling the sales tax on the receipt gives a quick tip estimate.
US sales tax runs roughly 6–10% in most states, so doubling it lands close to the historical 15–20% tipping range — a genuinely fast mental-math shortcut, though it can undershoot the 18–20%+ baseline several 2026 etiquette guides now cite for ordinary good service.
Only where sales tax happens to sit near 8–10%. In Oregon there is no sales tax at all, so doubling nothing tips nothing; in parts of Louisiana combined rates exceed 11% and doubling over-tips. It is a shortcut about one country's tax rates, not about tipping.
At 8.875% the rule says 17.75% and the exact answer is 20% — 11% low. It holds to within 15% above 8.5%, and drifts below it.
It only lands near 20% where the tax is around 10%. In a low-tax state doubling the tax is a mean tip, and the rule gives no hint that it has stopped working.
The rule against the exact answer, computed across the range. Inside the shaded band the shortcut is close enough to use; outside it, reach for the calculator.
Estimate with the rule, then check it against the calculator that models it properly.
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