How to Convert Units Online for Free (2026 Guide)

Miles to kilometers, Fahrenheit to Celsius, pounds to kilograms, gigabytes to megabytes — unit conversions pop up in recipes, travel, fitness, tech specs and schoolwork. The formulas are simple but the constants are easy to misremember (is a mile 1.6 km or 1.9?). A free online unit converter handles six categories with exact factors built in. This guide shows you how to use each one, with worked examples you can double-check.

Step 1: Open the Unit Converter

Open our Unit Converter. Free, no signup, runs entirely in your browser.

Step 2: Pick a category, value and units

Four controls:

• Category — Length, Weight, Temperature, Volume, Speed or Data
• Value — the number to convert (pre-filled with 1)
• From — the unit you're converting from
• To — the unit you're converting to

Changing the category updates the From/To dropdowns automatically. Click "Convert" for the instant answer.

Length: miles to kilometers

Category = Length (units: m, km, cm, mm, mi, yd, ft, in). Example: a 5-mile run → kilometers:

• Value = 5, From = mi, To = km → 8.05 km (1 mile = 1.60934 km)
• Reverse check: 8.05 km → mi gives back ≈ 5 — the tool converts both ways.

Everyday uses: road-trip distances abroad, treadmill settings, height in feet vs. centimeters (6 ft = 182.88 cm), fabric and construction measurements.

Weight: pounds to kilograms

Category = Weight (units: kg, g, lb, oz, t). Example: a 180 lb person → kilograms:

• Value = 180, From = lb, To = kg → 81.65 kg (1 lb = 0.453592 kg)

Everyday uses: gym weights, luggage limits (23 kg ≈ 50.7 lb — handy before the airport scale), cooking ingredients, parcel shipping weights.

Temperature: Fahrenheit to Celsius

Category = Temperature (units: Celsius, Fahrenheit, Kelvin). Example: a 100°F fever → Celsius:

• Value = 100, From = Fahrenheit, To = Celsius → 37.78 °C
• Formula behind it: (°F − 32) × 5/9 — the tool applies it exactly.

Memorize these anchors and you'll rarely need the tool: 32°F = 0°C (freezing), 98.6°F = 37°C (body temp), 212°F = 100°C (boiling). Kelvin (used in science) just adds 273.15 to Celsius.

Volume: gallons to liters

Category = Volume (units: l, ml, gal, cup). Example: a 2-gallon fuel can → liters:

• Value = 2, From = gal, To = l → 7.57 liters (1 US gallon = 3.78541 l)

Everyday uses: US recipes (1 cup = 236.588 ml) cooked from a metric kitchen, fuel economy conversions, aquariums and cooking oil quantities.

Speed: mph to km/h

Category = Speed (units: km/h, m/s, mph). Example: 65 mph speed limit → km/h:

• Value = 65, From = mph, To = km/h → 104.61 km/h

Everyday uses: driving abroad, running pace (a 6 min/km pace ≈ 10 km/h), wind speeds in weather reports.

Data: gigabytes to megabytes

Category = Data (units: B, KB, MB, GB). Example: a 2 GB video file → megabytes:

• Value = 2, From = GB, To = MB → 2,048 MB (1 GB = 1,024 MB)

Note: this tool uses the binary convention (1,024-based) that operating systems and memory specs use. Storage manufacturers sometimes advertise decimal gigabytes (1,000-based) — that's why a "1 TB" drive shows as ~931 GB in your OS. Same bytes, different ruler.

Why the world has two measurement systems

Quick context that makes conversions less annoying: most of the world uses the metric system (meters, kilograms, Celsius) because it's decimal and consistent — 1,000 meters to a kilometer, 1,000 grams to a kilogram. The US customary system (miles, pounds, Fahrenheit) survives mainly in the United States, with the UK mixing both. That's exactly why converters exist: recipes, car specs, weather and fitness content cross these borders constantly. You don't need to memorize every factor — that's the tool's job — but knowing which system a number belongs to tells you whether a conversion is even needed. When in doubt, convert; a 5-minute check beats a ruined recipe or a speeding ticket.

Tips for conversion confidence

• Sanity-check with anchors: 1 mile ≈ 1.6 km, 1 kg ≈ 2.2 lb, 1 inch = 2.54 cm exactly. If the tool's answer is far from your anchor estimate, re-check the From/To selection.
• Watch unit variants: US vs. imperial gallons differ (the tool uses US gallons); "ton" can mean metric tonne, US short ton or imperial long ton depending on context.
• Temperature needs the right formula: unlike other units, temperature has an offset (32°F = 0°C), not just a multiplier — the tool handles this, but DIY converters often don't.
• Precision vs. accuracy: the tool gives exact math; your input's precision is the real limit — converting a roughly-measured 5 miles to 8.0467 km doesn't make it more accurate.

Frequently asked questions

Is the unit converter free?

Yes — free, unlimited, no signup, computed in your browser.

How many categories and units are supported?

Six categories: Length (8 units), Weight (5), Temperature (3), Volume (4), Speed (3) and Data (4).

Why does 1 GB show as 1,024 MB and not 1,000?

The tool follows the binary (1,024-based) convention used by operating systems for RAM and file sizes. Drive makers use decimal (1,000-based) for marketing — both are "correct" under different standards.

Can I convert within the same system, like cm to m?

Absolutely — metric-to-metric conversions are the simplest and always exact (100 cm = 1 m).

Convert anything: Free Unit Converter — six categories, exact factors, zero signup.