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Convert Celsius to Reaumur - Temperature Converter

This tool converts a single temperature value from degrees Celsius (°C) to degrees Réaumur (°Ré). The Réaumur scale is a linear temperature scale historically used in parts of Europe; its defined ratio to Celsius makes conversions straightforward and exact with basic arithmetic.

Use this converter for quick calculations, lab note conversions, recipe adjustments, and documentation where the Réaumur scale is referenced. For metrology or legal requirements, ensure your measured temperatures are obtained from calibrated instruments traceable to recognized standards.

Updated Nov 23, 2025

Interactive Converter

Convert between celsius and reaumur with precision rounding.

Quick reference table

CelsiusReaumur
1 °C0.8 °Ré
5 °C4 °Ré
10 °C8 °Ré
25 °C20 °Ré
50 °C40 °Ré
100 °C80 °Ré

Methodology

The Réaumur scale is defined so that the freezing point of water is 0°Ré and the boiling point of water at standard pressure is 80°Ré. Celsius is defined with freezing 0°C and boiling 100°C. Because both scales are linear and share the same zero for the freezing point of water, the conversion is a simple linear scaling.

For measurement-grade work, conversions assume temperatures have been measured according to recognized protocols (for example ITS-90 and instrument calibration). Converting a recorded temperature is a mathematical operation that does not change measurement uncertainty; always propagate instrument uncertainty when reporting converted values.

Worked examples

Example 1: 100 °C → 100 × 4/5 = 80 °Ré

Example 2: -10 °C → -10 × 4/5 = -8 °Ré

Key takeaways

Convert °C to °Ré by multiplying by 4/5 (0.8). Reverse by multiplying °Ré by 5/4 (1.25). The arithmetic is exact; reported precision should reflect measurement uncertainty and calibration status.

Further resources

Expert Q&A

Is the conversion exact?

Mathematically the conversion factor (4/5 or 0.8) between Celsius and Réaumur is exact because both are linear scales with fixed ratios. However, reported converted values inherit the measurement uncertainty of the original temperature reading. For traceable measurements, follow NIST/ISO-recommended calibration and uncertainty reporting procedures.

Do I need to calibrate instruments before converting values?

Calibration is a property of the measuring instrument, not the conversion. If your results must comply with metrology or regulatory requirements, ensure instruments are calibrated and traceable to standards such as ITS-90. Converting calibrated readings preserves the numeric transformation but does not reduce measurement uncertainty.

What rounding or precision should I use?

Choose significant digits consistent with the original measurement uncertainty. For everyday use, two decimal places is usually sufficient. For laboratory or legal reporting, follow your lab's uncertainty and significant-figure policies and document the uncertainty alongside the converted value.

Are there regulatory references I should follow for temperature measurement?

Yes. For metrology and calibration use NIST guidance on temperature and ITS-90. ISO standards cover quantities and units for thermodynamics. Occupational exposure limits and workplace temperature guidance can come from OSHA. Consult the specific standards applicable to your industry when measurements are used for compliance.

Can I convert extremely large or small temperatures with this formula?

The linear conversion works across the numeric range of the scales, but practical limits depend on measurement method and instrument range. For extreme temperatures near instrument limits or requiring thermodynamic temperature definitions, refer to ITS-90 and instrument manufacturer specifications.

Sources & citations