Boiling Water Evaporation Rate Calculator for Food

This calculator estimates the average rate at which water leaves a cooking pot while it boils, using the water mass before and after a measured interval. Enter the starting mass, ending mass, and elapsed time to get an evaporation rate in grams per hour; it estimates water loss, not a guaranteed recipe finish time.

Formula

The average evaporation rate is the decrease in water mass divided by the elapsed time:

R = (M₁ − M₂) × 60 / T

• R is the average rate in grams per hour (g/h).
• M₁ is the starting water mass in grams (g).
• M₂ is the ending water mass in grams (g).
• T is the elapsed boiling time in minutes; multiplying by 60 converts the per-minute result to per hour.

The calculator also reports the observed mass lost and converts the rate to ounces per hour, using 1 ounce = 28.3495 grams. The inputs must describe a real decrease in mass and a positive time. If the ending mass is greater than the starting mass, the calculator cannot produce a meaningful evaporation estimate from those measurements.

Worked Example

Using the defaults: starting water mass M₁ = 1,000 g, ending water mass M₂ = 900 g, and boiling time T = 30 minutes.

1. Mass lost = 1,000 − 900 = 100 g.
2. Rate = 100 × 60 / 30 = 200 g/h.
3. In ounces per hour: 200 / 28.3495 = 7.05 oz/h, rounded to two decimals.

The calculator therefore returns an average evaporation rate of 200.0 g/h and a water loss of 100.0 g.

What the Rate Means in Cooking

Evaporation removes water from a pot and can concentrate soups, sauces, stocks, and other foods. A rate is useful for comparing a particular pot and burner setup, or for estimating how much water may disappear during a longer simmer. It is an average over the measured interval, not a constant physical property of the pot.

For example, if a measured rate is 200 g/h, a simple estimate for 45 minutes is 200 × 45 / 60 = 150 g of water. This projection is only reasonable when the cooking conditions stay similar. As the liquid level, temperature, food composition, and burner setting change, the rate can change too. The estimate should not be treated as a precise amount to add or remove from a recipe.

One gram of water is approximately one milliliter at ordinary kitchen temperatures, so 200 g is roughly 200 mL, or about 0.85 US cup. That is a convenient approximation for water, not a general conversion for oil, flour, or other ingredients. Mass measurements are more dependable than volume measurements when comparing evaporation.

How to Measure It Reliably

Use a scale and a repeatable method. A pot’s weight can be substantial, so check that the scale can handle the pot safely and that the entire pot can sit stably on it. Do not place a hot pot on a scale unless the scale is designed for the temperature and load; a safer approach is to measure the water before heating and after cooling, with the pot’s tare handled consistently.

1. Measure the water mass before boiling. Record the mass of the water itself, or record the filled pot and subtract the pot’s empty mass.
2. Bring the water or recipe to the intended boil or simmer, and start timing only when the chosen cooking condition is reached.
3. Keep the lid position, burner setting, and pot unchanged during the interval. Note any splashing or liquid removed for tasting.
4. Stop the timer, cool the pot enough for safe weighing, and measure the remaining contents using the same weighing method.
5. Use the result as a rate for those conditions, then repeat the measurement if you need a more representative estimate.

In food containing solids, the change in total pot contents is not automatically water evaporation. Steam is mostly water, but ingredients may release water, absorb it, or lose other volatile compounds. Spills, boiling over, sampling, and food stuck to a utensil also affect the weight. For a clean measurement, plain water is easiest; for a recipe, interpret the result as an approximate net mass change.

What Changes Evaporation

• Surface area: A wide pot exposes more liquid surface than a narrow pot, often increasing evaporation under otherwise similar conditions.
• Heat input and boil intensity: A vigorous boil generally moves more vapor away from the surface, but burner settings are not directly comparable across cookers.
• Lid position: An uncovered pot loses more vapor to the room. A lid can reduce net water loss, although steam may escape around its edges.
• Air movement and room conditions: Moving air can carry vapor away; temperature, humidity, and altitude can also affect boiling and vapor transfer.
• Liquid and food composition: Salt and dissolved ingredients alter boiling behavior, while food can thicken a liquid or change how it circulates.

Water boils at a lower temperature at higher elevations because atmospheric pressure is lower. That does not mean evaporation can be predicted from boiling temperature alone: exposed area, heat supplied, airflow, and condensation under a lid matter as well.

Common Mistakes and Limits

Do not enter a time in hours when the calculator expects minutes. For instance, 1.5 hours must be entered as 90 minutes. Keep both mass readings in grams; if using a scale set to ounces, convert first or use the displayed ounces-per-hour output only after calculating with gram inputs. Avoid rounding the two mass readings too aggressively, especially for a short test where the loss is small.

A measurement interval should be long enough that the mass change is larger than the scale’s display increment, but not so long that cooking conditions or ingredients change substantially. A short test can be distorted by scale resolution; a long test can conceal changes in boil intensity. Repeat tests and compare similar intervals to understand normal variation.

This calculator does not determine how long a food must cook, whether it is safe to eat, or how much liquid a recipe should contain. Evaporation is only one part of cooking. Follow recipe directions and applicable food-safety guidance; do not use a mass-loss result as a substitute for checking a dish’s required internal temperature or safe handling.

Frequently Asked Questions

How much water evaporates from a pot per hour?
There is no universal rate. It depends on pot diameter, heat, lid position, airflow, altitude, and the liquid. Weigh the water over a timed interval in your own setup to estimate its average rate.

Can I calculate evaporation from a recipe’s starting and ending volume?
You can estimate it, but volume readings are less reliable because hot liquids expand, foam obscures the level, and ingredients take up space. Mass measurements are usually more consistent. For water, 1 g is approximately 1 mL.

Does covering a pot stop evaporation?
No. A lid can reduce loss by trapping steam and allowing some to condense and return to the pot, but vapor can still escape around the lid or through a vent.

Can I use this calculator for soup or sauce?
Yes, as an approximate net mass-loss estimate, provided you weigh consistently and do not remove or spill food during the test. Ingredients can release or absorb water, so the result may not represent pure water evaporation.

Why does my measured rate change during cooking?
The liquid level, temperature, food thickness, exposed surface, and burner setting can shift over time. The formula gives an average for the interval measured, not a fixed rate for the entire recipe.

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in m1 | Starting water mass | g | 1000 | 0
in m2 | Ending water mass | g | 900 | 0
in t | Boiling time | min | 30 | 0
out r | Evaporation rate | g/h | (m1 - m2) * 60 / t + 0 * sqrt(m1 - m2) | 1
out rh | Evaporation rate | oz/h | r / 28.3495 | 2
out lost | Water lost | g | m1 - m2 + 0 * sqrt(m1 - m2) | 1