ConvertPlus

Free Online Unit & Temperature Converters

Instant, accurate conversions for temperature, Roman numerals, length, weight, and volume. No sign-up, no clutter — just answers.

40°C = 104°F

Browse by Category

Every converter you need, organized by topic.

Temperature

35 converters

Celsius ↔ Fahrenheit for every value that matters: body temperature, weather, and oven settings.

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Roman Numerals

15 converters

Numbers ↔ Roman numerals, dates, and the full 1–100 chart with the rules explained.

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Length & Weight

10 converters

The everyday unit pairs: kg ↔ lbs, cm ↔ feet, inches, km ↔ miles, and ml ↔ oz.

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Popular Conversions

The conversions people look up most.

40 Celsius to Fahrenheit100 Fahrenheit to Celsius25 Celsius to Fahrenheit37 Celsius to Fahrenheit180 Celsius to Fahrenheit350 Fahrenheit to Celsius104 Fahrenheit to Celsius200 Celsius to FahrenheitRoman Numeral ConverterRoman Numeral Date ConverterRoman Numerals 1–100XXXVII Roman NumeralsMMXXVI Roman NumeralsMCMXCVII Roman NumeralsKilograms to PoundsPounds to KilogramsCentimeters to FeetCentimeters to InchesKilometers to MilesMilliliters to Ounces

Why ConvertPlus?

Instant results

Every conversion is computed live in your browser as you type. No page reloads, no waiting, no 'Convert' button required.

Accurate formulas

We use the exact internationally defined conversion factors — like 1 inch = 2.54 cm exactly — and show the formula behind every answer.

Free, forever

No sign-up, no paywall, no data collection. Converters run entirely on your device, which also makes them fast and private.

Frequently Asked Questions

Is ConvertPlus free to use?

Yes — every converter on ConvertPlus is completely free, with no sign-up and no usage limits. All calculations run locally in your browser.

What is the formula for Celsius to Fahrenheit?

Multiply the Celsius value by 9/5 and add 32. For example, 40°C × 9/5 + 32 = 104°F. Our Celsius to Fahrenheit converter applies this instantly in both directions.

How do I read Roman numerals?

Symbols add from left to right (XVI = 10+5+1 = 16), except when a smaller symbol precedes a larger one — then it subtracts (IX = 10−1 = 9). The symbols are I=1, V=5, X=10, L=50, C=100, D=500, M=1000.

How many ounces are in a cup?

One US cup is 8 US fluid ounces, about 237 ml. For cooking conversions between metric and US measurements, try the ml to oz converter.

Do the converters work on mobile?

Yes. ConvertPlus is built as a fast, lightweight static site that works offline-friendly on any phone, tablet, or computer with a browser.