Speed, Power and Pressure: km/h, mph, kW, hp, bar and psi

Speed units: km/h, mph, knots and m/s

In Lithuania speed is given in kilometres per hour (km/h). In the US and UK, road signs use miles per hour (mph). One mile is exactly 1609.344 m, so 1 mph = 1.609344 km/h.

Seafaring and aviation use knots: 1 knot is one nautical mile per hour, exactly 1.852 km/h. In physics and engineering the usual unit is m/s, and 1 m/s = 3.6 km/h.

Speed limits abroad

When you drive in the US or UK, do not mix up the signs: 30 mph is about 48 km/h, 50 mph about 80 km/h, 65 mph about 105 km/h and 70 mph about 113 km/h. A quick estimate: mph × 1.6 = km/h.

Some cars show both units on the speedometer, and the smaller numbers are often the mph ones. If you are not sure, read the label near the dial and check which unit your speed is displayed in before you set off.

Power: kW and horsepower

Car power in Europe is often given in kilowatts (kW) and horsepower. But there are several kinds of horsepower. Mechanical horsepower (hp, used in the US and UK) is about 745.7 W, so 1 kW ≈ 1.341 hp. Metric horsepower (PS in German, CV in French) is about 735.5 W, so 1 kW ≈ 1.36 PS and 1 PS ≈ 0.7355 kW.

The difference is small but real: 100 kW is about 134.1 hp, but about 136 PS. European registration documents usually show kW, while sellers' adverts show PS. American manufacturers quote hp, so check first that a figure is not PS.

• 100 kW ≈ 134.1 hp ≈ 136 PS
• 110 kW ≈ 147.5 hp ≈ 149.6 PS
• 100 PS ≈ 73.55 kW
• 200 hp ≈ 149.1 kW

Pressure units: bar, kPa, psi and atm

Exactly, 1 bar = 100 kPa = 100,000 Pa. The standard atmosphere (atm) is 101.325 kPa = 1.01325 bar. 1 bar ≈ 14.5038 psi, and 1 psi ≈ 0.0689 bar (6.895 kPa). Meteorology uses hectopascals (hPa): 1 hPa = 1 millibar, so 1013 hPa is about 1.013 bar.

In your head: psi ÷ 14.5 = bar or, more roughly, psi ÷ 15. For example, 30 psi ≈ 2.07 bar. To go from bar to psi, multiply by 14.5.

Car tyre pressure

European car information stickers show tyre pressure in bar, while US cars show psi. For example, 2.2 bar is about 32 psi, and 35 psi is about 2.4 bar. Most compressors have both scales, but check which scale shows which value.

Always take the correct pressure from the car maker's sticker (often on the door frame or fuel flap), not from the number moulded on the tyre: that one is the maximum permitted pressure, not the recommended one.

Air pressure and weather

Weather forecasts in Lithuania give pressure in hPa or mm Hg. Standard sea-level pressure is 1013.25 hPa, about 760 mm Hg. American apps may use inHg: 1013.25 hPa is about 29.92 inHg. Values well below about 1000 hPa are usually associated with cyclones and wetter weather, and values above about 1020 hPa with clearer weather.

Worked examples

Speed: 75 mph × 1.609344 ≈ 120.7 km/h, and 120 km/h × 0.621371 ≈ 74.6 mph. A ship travelling at 20 knots moves at about 37.04 km/h (20 × 1.852). Power: 85 kW × 1.34102 ≈ 114.0 hp, while 85 kW × 1.35962 ≈ 115.6 PS. In reverse, 150 PS × 0.73550 ≈ 110.3 kW.

Pressure: 2.3 bar × 14.5038 ≈ 33.4 psi. On a bicycle tyre, 6 bar is about 87 psi. In a weather app, 29.80 inHg is about 1009 hPa (29.80 × 33.8639 ≈ 1009.1).

Common mistakes

The first mistake is confusing kW with kWh. A kilowatt measures power (how fast), a kilowatt-hour measures energy (how much in total). Both numbers often appear side by side in descriptions of electric cars and heaters.

Second: pressure in bar usually means gauge pressure, that is, above the surrounding air pressure. Technical specifications sometimes write barg (gauge) or bara (absolute), and they differ by about 1 bar. Third: measure tyre pressure on cold tyres, because it rises after driving. Fourth: a car speedometer may show mph on its main scale, so find out which scale is which before you set off.

Checklist

• Is the speed in km/h, mph or knots?
• Is the horsepower hp or PS?
• Is the pressure in bar, psi, kPa or atm?
• Have you avoided mixing up power (kW) and energy (kWh)?
• Are you measuring tyre pressure on cold tyres?