Hydrometer Temperature Correction Calculator
Correct your hydrometer readings for temperature. Supports SG and Plato/Brix with scientific-grade water density formulas.
How to use
Choose your reading unit, enter the observed hydrometer value, and supply the liquid temperature that was measured at the time. Select the hydrometer’s calibration temperature (20 °C or 60 °F) or provide a custom calibration to compute the correction.
Methodology
The tool converts SG ↔ Plato as needed, computes the water density ratio at the sample vs. calibration temperature using the Kell (1975) polynomial, and multiplies the observed SG by that ratio to produce the corrected gravity that would be observed at the calibration temperature.
Full original guide (expanded)
Data Source and Methodology
The calculator is grounded in Kell’s 1975 water-density polynomial and the IAPWS-95 reference for fluid density. These formulations define ρ_w(T), the density of pure water, which scales the specific gravity correction.
All calculations are strictly based on the formulas and data provided by those references.
The Formula Explained
The corrected specific gravity equals the observed SG times the ratio of water densities at the sample and calibration temperatures. The Kell polynomial expresses density as a fifth-order function of Celsius temperature, so we convert Fahrenheit inputs before evaluating it. Plato/Brix readings are converted to SG before correction and converted back for reporting.
Glossary of Variables
- SG_obs: Observed specific gravity at the measured sample temperature.
- T_obs: Sample temperature when the observation was made.
- T_cal: Hydrometer calibration temperature (20 °C, 60 °F, or custom).
- ρ_w(T): Water density at temperature T (°C) in kg/m³.
- SG_corr: Temperature-corrected specific gravity.
- P (°P): Degrees Plato/Brix, convertible to/from SG.
How It Works: A Step-by-Step Example
With SG 1.050 observed at 30 °C and a 20 °C calibration, the density ratio ρ_w(30)/ρ_w(20) ≈ 0.99744. Multiply 1.050 by the ratio to get SG_corr ≈ 1.0475, then convert to Plato (≈ 11.7 °P).
Frequently Asked Questions
Do I need to correct if my sample matches the calibration temperature? No; the ratio is 1 and the reading stays unchanged.
Can I enter readings in Plato/Brix? Yes. The calculator converts Plato to SG internally for the density correction, then reports both SG and °P outputs.
Is this valid for seawater? The correction uses pure-water densities, consistent with brewing and winemaking guidance. Use specialized tools for extreme salinity.
What accuracy can I expect? The Kell polynomial is accurate from 0 °C to 100 °C. Accuracy also depends on thermometer precision and hydrometer technique.
Why does corrected SG decrease at high temperatures? Warm water is less dense, so the hydrometer reads higher; the correction scales it back down.
Does the calculator consider hydrometer stem expansion? No. The dominant effect is water density, so stem expansion is negligible and excluded.