5 min read · Free educational guide
Anchor on 10% and 1%
10% moves the decimal one place left ($64.50 → $6.45). 1% moves it two places ($64.50 → $0.645). Almost every everyday percent can be built from those anchors.
Useful combinations
5% is half of 10%. 15% is 10% + 5%. 20% is double 10%. 25% is a quarter of the whole. 50% is half. 75% is half plus a quarter.
When to switch to a calculator
Messy decimals, stacked discounts, loan amortization, and compound growth deserve a tool. Use mental math to spot-check, not to replace careful work on high-stakes numbers.
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FAQ
- How do I estimate 18% tip quickly?
- Take 20% (double 10%) then subtract a little, or do 10% + 5% + 3% (3% ≈ a third of 10%).