What to convert before you land
The sneakiest travel mistakes are the ones that feel right in your head and are wrong on the ground. A "90" on a German autobahn sign is 90 km/h (56 mph), not 90 mph — two very different cars worth of energy. A 23 kg baggage allowance is 50 lb on the nose in North American bookings, but your home scale in lb is not the same as the airport scale in kg unless you do the multiplication. A "30" hotel room is comfortable in Dublin (°C, shirtsleeves) and a fainting risk in Phoenix (°F, subtract thirty).
The three conversions that cover 90% of everyday travel friction are: temperature (°C ↔ °F), distance (km ↔ mi), and weight (kg ↔ lb). Memorize one rough multiplier for each, use this cheat sheet for the rest, and you'll spend less mental energy on units and more on the trip itself.
Driving abroad — the unit shift that matters most
Speed, distance, and fuel are the three units you'll be staring at for hours behind the wheel. North American drivers in Europe habitually underestimate their speed — "80" feels slow until you remember it's 80 km/h, i.e. 50 mph. European drivers in the US often overestimate — "70" on a Nevada freeway is 70 mph, 112 km/h, and closing distance to the next car faster than your reflexes are calibrated for. The first hour of driving in unit-swapped territory is the riskiest; pay attention to the speedometer more than usual.
Fuel economy flips too. A rental in France will be quoted in L/100km, a rental in Texas in MPG, and a rental in Tokyo in km/L. They are all rankable, but only if you convert. Our fuel-economy converter sits right next to this page for exactly that reason.
Temperature — the conversion most people get backwards
The formula F = C × 9/5 + 32 is correct but hard to do in a crowded airport. Three shortcuts that survive distraction:
- Double and add 30. 20 °C → 70 °F (real 68). 25 °C → 80 °F (real 77). 30 °C → 90 °F (real 86). Fast, accurate enough for packing.
- Four landmarks. 0 = 32 (freezing). 10 = 50 (jacket). 20 = 68 (pleasant). 30 = 86 (hot). Interpolate between.
- The 28/28 trick. -40 °C = -40 °F (exact — the two scales cross there). 28 °C = 82 °F (inside 1°). Handy for arctic trips and summer Madrid alike.
If you want context for what a temperature means in daily life — "is 38 °C a heatwave or just a warm afternoon?" — see the temperature by feel guide.
Currency — when to change money, when to tap a card
As of 2026 the right answer is almost always a tap-to-pay card that does not charge foreign transaction fees. Airport exchange booths post the worst rates on earth, often 8–12% worse than interbank; hotel "convenient exchange" is worse still. Pull a small amount of local cash from an ATM inside the airport (use a card that reimburses ATM fees), and run every other transaction on a no-foreign-transaction-fee Visa or Mastercard. Amex coverage is spotty outside North America, Japan, and tourist-heavy European cities.
When the card terminal offers "dynamic currency conversion" — asking if you want to be charged in your home currency or local — always pick local. The "helpful" home-currency option is a 3–8% markup on top of your bank's already-competitive mid-market rate. Our currency converter uses reference rates from January 2026; for live rates use your banking app.
Voltage and plugs — the one gotcha that can fry your devices
Two independent things vary by country: plug shape and voltage. Plug adapters are cheap, light, passive — they reshape the pins so your device fits the socket. Voltage converters are heavy, active, and only needed if your device is not dual-voltage. Check the label — "Input: 100–240V" means you only need the adapter. "Input: 120V" means you need a real converter or you will smell expensive smoke.
Frequency (50 vs 60 Hz) matters for clocks built into small appliances and for some motorized tools; for chargers and electronics it is almost never an issue. Japan is the curious case — 50 Hz in the east (Tokyo), 60 Hz in the west (Osaka) — a legacy of 19th-century German and American equipment imports.
Airline baggage — the weight limit that isn't really a limit
The "23 kg / 50 lb" checked bag limit and "7 kg / 15 lb" carry-on limit are industry conventions, not physics. Enforcement varies wildly: busy airports, strict carriers (budget airlines, low-cost Asian routes) weigh aggressively; many full-service carriers wave through a 24 kg bag without comment if the line is long. Be ready to pay, be ready to move things into the carry-on, and remember that most airlines weigh the checked bag but rarely the carry-on unless it looks big. Pack dense items (shoes, books) in the carry-on and fluffy items (jackets, towels) in the checked bag when you're at the limit.
If your home scale reads pounds and you're flying a carrier that limits kilograms, do the math at home, not at the check-in counter. Use the weight converter with the preset buttons for 23 kg / 50 lb / 7 kg.
Tipping — the custom that most confuses Americans abroad and visitors to America
In the US, 20% of the pre-tax bill is the honest answer for decent restaurant service; 18% is minimally acceptable; less than 15% is a statement. In most of Europe, service is included and an extra 5–10% for great service is a compliment, not an obligation. In Japan and South Korea, tipping can actively offend — leave exact change. The country panel above has the local norm.
More travel-adjacent tools
- Currency converter — 30+ world currencies.
- Length converter — miles to kilometers and back.
- Weight converter — airline baggage in kg and lb.
- Temperature converter — Celsius ↔ Fahrenheit.
- Fuel economy converter — MPG ↔ L/100km ↔ km/L.
- Shoe size converter — US, UK, EU, Japan.
- Clothing size converter — US, UK, EU, IT, FR.