Length: the conversions worth memorizing
The two most commonly needed length conversions are 1 inch = 2.54 centimeters (exact, by definition) and 1 mile = 1.609 kilometers. From there, 1 foot = 30.48 cm, and 1 meter is a bit longer than a yard (1 meter ≈ 1.094 yards) — a useful rough mental anchor even though it's not exact. For quick mental estimates, roughly 5 cm per 2 inches and roughly 60 mph ≈ 100 km/h are close enough for everyday, non-precise use.
Weight: the ones that come up most
1 kilogram ≈ 2.205 pounds is the conversion worth remembering — a quick mental shortcut is roughly doubling a kilogram figure to estimate pounds (a bit of an overestimate, but close for a fast gut check). 1 pound = 453.6 grams, and 1 ounce ≈ 28.35 grams, both useful for cooking or shipping-weight contexts where the unit switches between grams and ounces.
Convert length, weight, or volume
Volume: where recipes get confusing
Recipe conversions are where metric/imperial mixing causes the most everyday friction: 1 US cup ≈ 236.6 mL, 1 tablespoon ≈ 14.8 mL, and 1 teaspoon ≈ 4.9 mL. It's worth knowing that a US cup and a metric cup (used in Australia, the UK, and elsewhere) aren't quite identical — a metric cup is exactly 250 mL, while a US cup is closer to 237 mL — a small difference that can matter in a precise baking recipe even though it's negligible for most cooking.
Temperature: not a simple ratio
Unlike length, weight, and volume, temperature scales don't share a common zero point, so converting between them needs an offset, not just a multiplier. Celsius to Fahrenheit is (°C × 9/5) + 32; Fahrenheit to Celsius is (°F − 32) × 5/9. A quick mental approximation — double the Celsius figure and add 30 — gets close enough for a rough weather-check conversion, though it drifts further from the exact answer at extreme temperatures.