Dew Point Calculator.

Formula and practical context.

Formula, variables, and method

γ = ln(RH/100) + aT/(b+T); dew point = bγ/(a−γ). T is Celsius, RH is relative humidity, a = 17.625, and b = 243.04 °C for this Magnus approximation. Dew point is the temperature at which air with the entered moisture content would become saturated under the approximation. Start by identifying what each entered number represents and whether it is a measured value, a specification, or an assumption. Convert units before combining quantities, preserve full precision through intermediate steps, and round only the displayed result. Convert to Celsius, compute gamma from temperature and the natural logarithm of relative humidity, then solve the Magnus relation for dew point. This sequence makes the output reproducible and helps expose a misplaced unit or decimal point before it affects a decision.

Worked example and independent check

At 20 °C and 50% relative humidity, the Magnus approximation gives a dew point of about 9.3 °C. To check the example independently, write the formula first, substitute the values with their units, and complete one operation at a time. Compare the order of magnitude with a familiar reference rather than accepting a formatted result automatically. Convert to Celsius, compute gamma from temperature and the natural logarithm of relative humidity, then solve the Magnus relation for dew point. If a second method or inverse calculation is available, use it to recover an original input. Small differences from hand calculation should come only from displayed rounding, not from a different definition.

Units and result interpretation

Temperature may be entered in Celsius or Fahrenheit and is normalized to Celsius; relative humidity is a percentage from above zero through 100. Dew point is the temperature at which air with the entered moisture content would become saturated under the approximation. A unit is part of the quantity, not a label added after the arithmetic. Keep decimal and binary prefixes, temperature scales, elapsed-time units, and electrical prefixes explicit. When comparing two scenarios, change one assumption at a time and retain the same unit convention. The result describes only the relationship named by this calculator; it does not automatically establish capacity, safety, compatibility, quality, or compliance.

Validation and boundary cases

Relative humidity at zero is outside the logarithmic formula, humidity above 100% is invalid, and extreme temperatures outside normal meteorological use are rejected. Blank values, malformed text, non-finite numbers, impossible ranges, zero divisors, and unsupported combinations receive an explicit message rather than a silent fallback. Boundary values are checked before the formula runs. If an input comes from a specification sheet, weather report, network plan, or measurement instrument, confirm its definition and precision. A syntactically valid value can still be unsuitable for the real situation, so inspect the assumptions alongside the result.

Assumptions, privacy, and practical limits

The Magnus approximation is practical for common near-surface conditions but is not a substitute for calibrated observations or specialist psychrometric calculations. All entered values are processed in the browser and are not sent to a calculation service. Use the result as a transparent estimate or transformation, keep the source values with any saved result, and consult the linked authoritative material for definitions and applicability. For safety-critical, health-adjacent, security, network, chemical, or electrical work, verify the result with current standards, official warnings, qualified professionals, and appropriate test equipment.

Check assumptions and sources.

The Magnus approximation is practical for common near-surface conditions but is not a substitute for calibrated observations or specialist psychrometric calculations.

NOAA National Weather Service humidity guidance