The Unit Conversion Cheat Sheet
Last updated: 2026-06-25
Most unit conversions come down to "collect into one base unit, then divide back." But areas with traps — temperature (add/subtract) and data storage (1024 vs 1000) — have to be remembered separately.
This article gathers the conversions people get wrong most across length, weight, area, volume, temperature, speed, data, cooking and shoes, with key factors and mental-math shortcuts.
The core principle: collect into a base unit
Unit conversion feels hard, but the principle is single. Any quantity of the same kind (length, weight, volume and so on) can be converted into one "base unit," and once you collect everything into that base, you can convert to any unit you like. Use the meter (m) for length, the gram (g) for weight and the milliliter (mL) for volume. To turn inches into yards, you do not need a separate inch-to-yard formula — just convert inches to meters and then meters to yards. All of our converters work this way.
There are two exceptions. First, temperature places its zero differently for each scale, so multiplication alone will not convert it. Second, cases like shoe sizes have no clean formula and are matched with a standard lookup chart. Aside from these two, almost every conversion is "multiply and divide."
Length: cm and inches trip people up most
The conversion you meet most in daily life is between cm and inches. There is just one key number: 1 inch = 2.54 cm. Multiply by 2.54 to go from inches to cm, and divide by 2.54 for the reverse. For mental math, 2.5 is close enough — a 27-inch monitor is 27 × 2.54 ≈ 68.6 cm.
The same factor applies to heights in feet and inches. One foot is 12 inches (30.48 cm), so a height of 175 cm is 175 ÷ 2.54 ≈ 68.9 inches, about 5 feet 8.9 inches. For the larger mile, 1 mile = 1,609.344 m, so a 5 km run is about 3.1 miles.
Weight: kg, pounds and the Korean geun and don
The key to weight is 1 kg ≈ 2.2 pounds. Precisely, 1 pound = 453.59237 g, but for mental math multiply kg by 2.2 to get pounds. A body weight of 70 kg is about 154 pounds. Conversely, a 1-pound piece of meat in a US recipe is about 0.45 kg, roughly 450 g.
It also helps to know the Korean units: in butcher shops and markets, 1 geun is 600 g (375 g for herbal medicine), and for gold and silver 1 don is 3.75 g. A one-don gold ring is 3.75 g and ten don is 37.5 g. People also confuse the weight ounce (about 28.35 g) with the fluid ounce (about 29.57 mL) — they are entirely different units.
Area and volume: square and cube the length factor
This is where the biggest mistakes happen. For length, 1 m = 100 cm, but for area 1 m² = 10,000 cm² (100 squared) and for volume 1 m³ = 1,000,000 cm³ (100 cubed). Using the length factor directly for area or volume is wrong by 100× and 10,000×.
The pyeong, essential in Korean real estate, is 1 pyeong ≈ 3.3058 m². An exclusive area of 84 m² is 84 ÷ 3.3058 ≈ 25.4 pyeong. For mental math, multiply m² by 0.3 to get pyeong (84 × 0.3 ≈ 25). For volume, remembering 1 L = 1,000 mL = 1,000 cm³ and 1 m³ = 1,000 L covers most cases.
Temperature: multiplication alone is not enough
Temperature behaves differently from other units. Because 0 °C and 0 °F point to different temperatures, you cannot convert just by multiplying a ratio; you need addition and subtraction. The formulas are °F = °C × 9/5 + 32 and, in reverse, °C = (°F − 32) × 5/9. Body temperature 36.5 °C is 36.5 × 1.8 + 32 = 97.7 °F. The Kelvin used in science is simply K = °C + 273.15, with absolute zero 0 K at −273.15 °C. One fun fact: −40 °C and −40 °F are exactly the same.
Speed: anchor on km/h
Speed is easiest if you anchor on km/h: 1 m/s = 3.6 km/h, 1 mph ≈ 1.6 km/h and 1 knot = 1.852 km/h. A US road sign of 60 mph is about 96 km/h, and a ship at 20 knots does about 37 km/h. Mach is a ratio to the speed of sound, so remember Mach 1 ≈ 1,225 km/h (about 340 m/s) in standard atmosphere.
Data storage: the 1024 vs 1000 trap
Data storage is the most confusing because the same "GB" has two bases. Operating systems use powers of two, so 1 GB = 1,024 MB, while hard drive and USB makers and carriers use decimal, so 1 GB = 1,000 MB. That is why a 1 TB external drive shows as about 931 GB on a computer: dividing a trillion bytes by 1,024⁴ gives about 931. It is also easy to confuse bytes (B) and bits (bit), which differ by a factor of 8 — a 100 Mbps connection is about 12.5 MB per second.
Cooking and shoes: tables are the answer
Cooking conversion needs ingredient density to turn volume (cups, tablespoons) into weight (grams). On the metric standard, 1 cup = 200 mL, 1 tablespoon = 15 mL and 1 teaspoon = 5 mL, multiplied by densities such as water 1.0, flour 0.53, sugar 0.85 and salt 1.2 (g/mL). So the same 1 cup is 200 g of water but only about 106 g of flour.
Shoe sizes have no clean formula because countries measure differently. Korea states foot length in mm (Mondopoint), the US and UK use inch-based numbers, and Europe uses Paris points, so a standard lookup chart is the most accurate match. The same US 8 differs between men and women, so check the gender first.
Key factor summary table
| Field | Key factor | Mental-math shortcut |
|---|---|---|
| Length | 1 inch = 2.54 cm | cm ÷ 2.5 ≈ inches |
| Weight | 1 kg ≈ 2.2 lb | kg × 2.2 ≈ pounds |
| Area | 1 pyeong ≈ 3.3058 m² | m² × 0.3 ≈ pyeong |
| Volume | 1 L = 1,000 mL | 1 m³ = 1,000 L |
| Temperature | °F = °C × 1.8 + 32 | −40 °C = −40 °F |
| Speed | 1 m/s = 3.6 km/h | mph × 1.6 ≈ km/h |
| Data | 1 GB = 1,024 MB (binary) | makers use 1,000 MB |
Summary
- Most conversions are solved by collecting into a base unit and dividing back.
- Square or cube the length factor for area and volume.
- Temperature is the one exception that needs addition and subtraction.
- For data storage, distinguish the 1024 (OS) and 1000 (maker) bases.
- For cooking and shoes, density tables and lookup charts are the most accurate.
Frequently asked questions (FAQ)
Which unit conversions do people get wrong most?
Temperature and data storage. Temperature does not convert by multiplication alone — you have to add or subtract 32 — while data storage mixes a 1024 (binary) and a 1000 (decimal) base, so the same GB can differ.
Are there shortcuts for quick mental conversion?
Yes. Divide cm by 2.5 for inches, multiply kg by 2.2 for pounds, multiply m² by 0.3 for pyeong, and multiply m/s by 3.6 for km/h. Memorizing these approximate factors lets you estimate quickly.
How are area and volume different from length?
Square a length factor for area and cube it for volume. For example, 1 m = 100 cm, but 1 m² = 10,000 cm² and 1 m³ = 1,000,000 cm³. Using the length factor directly causes large errors.
Related converters
Last updated: 2026-06-25