---
title: "Speed Calculator — Speed, Distance & Time in mph, km/h, Knots"
description: "Calculate speed, distance or time. Convert between mph, km/h, m/s and knots. Learn which countries use mph vs km/h and why knots are used in aviation."
url: "https://worldcalculators.org/calculators/speed/"
canonical: "https://worldcalculators.org/calculators/speed/"
---

# Speed Calculator

Calculate speed, distance or time. Convert between mph, km/h, m/s and knots. Learn which countries use mph vs km/h and why knots are used in aviation.

## Quick answer

100 km/h = 62.1 mph = 27.8 m/s = 54.0 knots. Cruising at 60 mph for 2.5 hours covers exactly 150 miles (241 km).

## Speed Limits by Country

The UK and USA are the only major countries that use miles per hour for road speeds. The UK is officially metric for most measurements but legally required to use miles on road signs. The USA retained the imperial system largely due to infrastructure costs of changing signage — an estimated $500M to switch all road signs to metric.
Knots are used worldwide in aviation and maritime navigation because 1 nautical mile = 1 minute of arc of latitude, making navigation calculations much simpler.

## Frequently Asked Questions

The speed of sound in dry air at 20°C (68°F) is 343 m/s = 1,235 km/h = 767 mph = 667 knots. This is Mach 1. Aircraft are considered supersonic above Mach 1. The speed of sound varies with temperature and altitude — decreasing as altitude increases.
The speed of light in a vacuum is 299,792,458 m/s = 1,079,252,849 km/h ≈ 670,616,629 mph. It is the fundamental constant of the universe (c). GPS satellites must account for relativistic effects because light-speed signals take time to travel and clocks on the satellite tick at different rates.

Source: https://worldcalculators.org/calculators/speed/
Markdown mirror of the HTML page. Prefer this URL for RAG; the interactive calculator still lives on the HTML page.