Unit Conversion Cheat Sheet for Everyday Life
How many centimeters in an inch? How many cups in a liter? What is 72 degrees Fahrenheit in Celsius? These tiny questions pop up constantly, in recipes, travel, DIY projects, and online shopping, and most of us half-guess them.
This cheat sheet gathers the everyday conversions in one place with exact factors and the mental-math shortcuts that get you close enough in seconds. Bookmark it, or better, learn the five tricks at the end and retire the guessing.
US customary units are the focus, since the US is the odd one out still using them daily, with metric equivalents alongside. Exact factors are given to enough precision for real life, not laboratory work.
Length: inches, feet, miles, and metric
The anchor: 1 inch = 2.54 cm exactly. From there, 1 foot = 30.48 cm, 1 yard = 0.9144 m, and 1 mile = 1.609 km. Going the other way: 1 cm is about 0.39 inches, 1 meter is about 3.28 feet, and 1 km is about 0.62 miles.
Real uses: a 6-foot person is 183 cm (6 x 30.48). A 10k run is 6.2 miles (10 x 0.62). A 55-inch TV measures about 140 cm diagonally (55 x 2.54). Furniture from a European store: 200 cm is 6 feet 7 inches (200 / 30.48).
Mental trick: for miles to km, multiply by 1.6; for km to miles, multiply by 0.6. A 100 mph speed limit sign abroad means about 160 km/h; a 130 km/h limit means about 78 mph. Close enough for driving decisions.
Room dimensions are the classic relocation headache: a 12x15 foot bedroom is about 3.7 x 4.6 meters, or 16.7 square meters. European apartment listings in square meters convert by multiplying by 10.76 for square feet, so a 60 sqm flat is about 646 sq ft.
Weight: pounds, ounces, kilograms
The anchor: 1 pound = 0.4536 kg, and 1 kg = 2.2046 lb. For ounces: 1 oz = 28.35 g, and 1 g is about 0.035 oz. A useful pair: 1 lb is 16 oz, and 1 kg is 1,000 g.
Real uses: a 150 lb person weighs about 68 kg (150 x 0.4536). Luggage limited to 23 kg allows about 50.7 lb (23 x 2.2046), which is why the 50 lb airline limit and 23 kg limit are the same rule. A 12 oz steak is about 340 g.
Mental trick: halve the pounds and subtract a tenth to get kilos roughly. 150 lb: half is 75, minus 7.5 gives 67.5, very close to the true 68. For kilos to pounds, double and add 10%: 68 kg doubled is 136, plus 13.6 = 149.6.
Produce and postage use the small end: 500 g is just over a pound (1.1 lb), and a 2 kg parcel ships at about 4.4 lb. Baby weights cross systems constantly in parenting groups: a 3.5 kg newborn is about 7.7 lb, and 8 lb is about 3.6 kg.
Volume in the kitchen: cups, liters, gallons
US kitchen anchors: 1 cup = 236.6 mL, 1 tablespoon = 14.8 mL, 1 teaspoon = 4.9 mL, 1 fluid ounce = 29.6 mL. Bigger: 1 pint = 473 mL, 1 quart = 946 mL, 1 gallon = 3.785 L. And 1 liter = 4.23 US cups.
Real uses: a European recipe calling for 500 mL of stock needs just over 2 cups (500 / 236.6 = 2.11). A 2-liter soda bottle holds about half a gallon plus a cup (2 / 3.785 = 0.53 gal). A 750 mL wine bottle is about 25.4 fluid ounces.
Watch out for the imperial trap: a UK pint is 568 mL, not 473. And US and metric cup measures in recipes are close enough to swap, but for baking at scale, weigh ingredients instead: 120 g of flour beats any cup conversion.
Scaling recipes across systems is where weighing wins outright. Doubling a recipe written in cups doubles the conversion error too, but doubling grams is exact. If you bake often, a $15 kitchen scale eliminates an entire category of arithmetic from your life.
Temperature: the one formula to memorize
Celsius = (Fahrenheit - 32) x 5/9. Fahrenheit = Celsius x 9/5 + 32. That is the whole system; everything else is anchor points.
Memorize four anchors and interpolate: 32 F = 0 C (freezing), 68 F = 20 C (room temperature), 98.6 F = 37 C (body temperature), 212 F = 100 C (boiling). A 72 F day is a mild 22 C; a 40 C heatwave is a brutal 104 F.
Real uses: oven at 350 F is 177 C, so 180 C on a European recipe is right. A fever of 101 F is 38.3 C. Weather abroad: 25 C is a pleasant 77 F, and -10 C is a bitter 14 F.
Speed and distance on the road
1 mph = 1.609 km/h, so multiply mph by 1.6 for km/h and km/h by 0.62 for mph. A 65 mph highway limit is about 105 km/h; a 120 km/h European motorway is about 75 mph.
Fuel economy crosses systems too: 30 mpg (US) is about 7.8 L/100 km (compute 235.2 / 30). The formula L/100km = 235.2 / mpg works for any US mpg figure; 40 mpg becomes 5.9 L/100km.
Trip planning: 300 miles at 60 mph is 5 hours, and 300 miles is 483 km. When renting abroad, remember the car's range readout may be in km: 500 km of range is about 310 miles.
Five mental tricks that replace the cheat sheet
One: temperatures, double the Celsius and add 30 for a quick Fahrenheit estimate (20 C -> 70 F, close to the true 68). Two: miles to km, multiply by 8 and divide by 5 (60 mph -> 96 km/h, true 96.5).
Three: pounds to kilos, halve it (150 lb -> ~75, then shave a bit to 68). Four: inches to cm, multiply by 2.5 (10 in -> 25 cm, true 25.4). Five: for exact answers, do the full factor multiplication once and write it down; estimates are for decisions, precision is for construction and baking.
The pattern behind every trick: round the exact factor to something friendly, apply it, then nudge. 2.54 becomes 2.5, 1.609 becomes 1.6, 0.4536 becomes 0.45. Your error stays under 5%, which is plenty for everyday life.
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Try the Length calculatorFrequently asked questions
How many centimeters are in an inch?
Exactly 2.54 cm. For mental math, 2.5 is close enough.
How do I convert Fahrenheit to Celsius?
Subtract 32, then multiply by 5/9. So 77 F is (77-32) x 5/9 = 25 C. For a quick estimate, halve the Fahrenheit and subtract 15: half of 77 is 38.5, minus 15 is 23.5, close to the true 25.
How many kilograms in a pound?
0.4536 kg. Roughly halve the pounds for a quick estimate: 150 lb is about 68 kg.
How many milliliters in a US cup?
236.6 mL. A liter holds about 4.23 cups, and a tablespoon is 14.8 mL.
How many liters in a US gallon?
3.785 liters. A UK imperial gallon is larger at 4.546 liters, so check which gallon you mean. For fuel economy, 30 US mpg converts to about 7.8 liters per 100 km.
Why does the US still use imperial units?
Historical inertia plus the enormous cost of switching: road signs, building codes, manufacturing tooling, and everyday habits would all need converting. Science, medicine, the military, and most industries already use metric; everyday life, cooking, and construction mostly do not.