Weather unit converter
The United States is the only large country that still reports weather in Fahrenheit, miles per hour and inches of mercury, which makes reading a foreign forecast — or explaining an American one — an exercise in conversion. Pick a quantity, enter a value, and get every equivalent at once.
Temperature: why Fahrenheit survives in one country
Daniel Fahrenheit built his scale in 1724 around two reference points he could reproduce in a workshop: a freezing brine of water, ice and ammonium chloride at zero, and roughly human body temperature near the upper end. Anders Celsius took a cleaner approach two decades later, anchoring his scale to the freezing and boiling points of pure water at sea level.
Celsius won almost everywhere, but Fahrenheit has one genuine advantage for describing weather: its degrees are smaller, so ordinary conditions span a wider range of whole numbers. Comfortable outdoor temperatures run from about 50 to 90 °F, forty steps, versus 10 to 32 °C, twenty-two steps. That finer resolution is why Americans say 72 rather than 22.
Two conversion shortcuts are worth memorizing. To go from Celsius to Fahrenheit roughly, double it and add 30 — that is within a couple of degrees across normal weather. And the scales cross at -40°, the single temperature where both read the same, which happens often enough in Minnesota and Alaska to come up in conversation.
Pressure: inches of mercury versus millibars
Inches of mercury is a literal measurement. Evangelista Torricelli's 1643 barometer was a sealed glass tube standing in a dish of mercury, and atmospheric pressure pushed the column up about 30 inches at sea level. American forecasts still quote that height. Standard sea-level pressure is 29.92 inHg, equal to 1013.25 millibars.
The rest of the world uses millibars, or the identical hectopascal, which measures force per unit area rather than the height of a liquid column. Meteorologists everywhere, including in the US, work in millibars internally because pressure surfaces and weather models are defined that way — 500 mb is the standard mid-atmosphere chart.
For reading a barometer, the trend matters more than the value. Falling pressure means an approaching low and usually deteriorating weather; rising pressure means a building high and clearing skies. A drop of more than 0.06 inHg per hour signals a rapidly developing storm. For perspective, Hurricane Wilma in 2005 recorded 26.05 inHg, or 882 mb, the lowest Atlantic reading ever measured.
Rain, snow and the numbers that catch people out
Rainfall is depth, not volume, which is why an inch of rain means the same thing on a windowsill and across a county. Converting to volume gives figures that surprise people: one inch of rain on one acre is about 27,150 gallons, roughly 113 tons of water. On a typical 1,500 square foot roof, that same inch delivers around 935 gallons.
Snow is where conversions get slippery. The old rule of thumb was ten inches of snow per inch of liquid water, but real ratios range from about 5:1 for the heavy wet snow that falls near freezing to 40:1 or higher for the dry powder that falls at 5 °F. That is why two storms with identical liquid totals can produce four inches or forty, and why forecasters give snowfall as a range.
Visibility follows the same US-versus-metric split. American aviation and surface reports use statute miles, marine forecasts use nautical miles, and the rest of the world uses kilometers. A Dense Fog Advisory is issued at a quarter mile, which is about 400 meters.
The formula
Temperature °F → °C (F − 32) × 5/9 °C → °F (C × 9/5) + 32 °C → K C + 273.15 Wind speed 1 mph = 1.609344 km/h = 0.868976 knots = 0.44704 m/s 1 knot = 1 nautical mile per hour = 1.15078 mph Pressure 1 inHg = 33.8639 mb (hPa) = 3.38639 kPa standard sea level = 29.92 inHg = 1013.25 mb Precipitation and distance 1 inch = 25.4 mm exactly 1 mile = 1.609344 km = 5,280 ft
Every factor here is an exact definition rather than a measurement. The inch has been defined as precisely 25.4 millimeters since the international yard and pound agreement of 1959, which is why these conversions never carry rounding uncertainty of their own.
Practical tips
- For a fast Celsius-to-Fahrenheit estimate, double and add 30. For the reverse, subtract 30 and halve.
- Knots equal nautical miles per hour, and one nautical mile is one minute of latitude. That is why marine and aviation forecasts use them: they map directly onto a chart.
- A 10 mph wind is a light breeze; 25 mph makes an umbrella useless; 40 mph is a High Wind Warning in most of the country; 74 mph is hurricane force.
- Remember 29.92 inHg as standard pressure. Below 29.5 usually means unsettled weather is on the way; above 30.2 generally means a stable high is in charge.
- When reading a foreign forecast, check whether wind is a ten-minute average. It will read about 12% lower than the US one-minute standard for the same storm.
Frequently asked questions
Why does the US still use Fahrenheit?
Inertia, mostly. The Metric Conversion Act of 1975 made metric the preferred system but left adoption voluntary, so public-facing weather never switched. American meteorologists work in metric and Celsius internally and convert for broadcast.
Are millibars and hectopascals different?
No, they are numerically identical: 1 mb = 1 hPa. The name changed to fit SI conventions, and both appear in forecasts depending on the country and the age of the equipment.
How much snow equals an inch of rain?
It depends on temperature. Around 10 inches near freezing, as little as 5 for heavy wet snow, and 20 to 40 for very cold dry powder. Colder air holds less moisture, so the same liquid equivalent spreads into a much deeper, fluffier layer.
What is a "trace" of precipitation?
Less than 0.01 inches — measurable enough to see but too little for the gauge to register. It is recorded as T in climate data, which is why some days show rain but a total of zero.