Hay Box Cooker Calculator
Calculate heat retention time, temperature curve, and fuel savings for a hay box (retained heat) cooker based on pot size, insulation, and food mass.
About this calculator
A hay box (also called a wonder box or fireless cooker) works entirely on stored heat: you boil the pot on a stove as normal, then move it into a heavily insulated box, where it keeps cooking as it slowly cools rather than needing continuous fuel. This calculator models that cooldown as Newtonian cooling — the pot's temperature decays exponentially toward room temperature, at a rate set by the "time constant" τ, which is the pot-plus-food's thermal capacity (mass times specific heat, using ~3.9 kJ/kg·°C for food and water plus a small fixed steel-pot contribution) divided by how fast heat escapes through the insulation (the inverse of its R-value, times the pot's estimated cylindrical surface area). From that decay curve, the calculator solves for how long the food stays above 75°C — a safe zone for continued slow cooking of stews and grains — and above 60°C, the food-safety floor below which food shouldn't linger, plus the actual temperature at the 2-, 4-, and 8-hour marks for meal planning.
The reported fuel savings (a fixed 83%, from comparing a 20-minute hay-box boil against a 2-hour stovetop simmer) is a rule-of-thumb estimate for a stew-type dish, not a calculation from your specific inputs — it comes out the same regardless of pot size, mass, or insulation, since only the cooling-curve outputs above respond to what you enter. Because the model assumes a fixed pot geometry and ignores lid seals or box air gaps, real-world retention will vary — but the relative effects of more food mass, a smaller pot, or better insulation all point the same direction the calculator shows.
Inputs
Results
Time above 75 °C
17.7 hours
How to Use This Calculator
- Enter the mass of food plus liquid in the pot (kg) and pot diameter (cm).
- Set the starting temperature (typically boiling, 100 °C) and insulation R-value.
- Enter ambient room temperature.
- The calculator shows how long food stays above 75 °C (safe cooking zone) and above 60 °C, plus temperatures after 2, 4, and 8 hours.
- Bring food to a full boil on your stove, then transfer immediately to the insulated box for hands-free retained-heat cooking.
How the result changes with Pot diameter
| Pot diameter | Time above 75 °C |
|---|---|
| 12 | 70.9 hours |
| 18 | 31.5 hours |
| 36 | 7.9 hours |
| 40 | 6.4 hours |
What each input means
- Food + liquid mass
- Total weight of food and liquid in the pot. More mass retains heat longer.
- Pot diameter
- Outer diameter of the cooking pot. Smaller pots cool faster.
- Starting temperature
- Temperature when the pot goes into the hay box. Should be at or near boiling (100 °C).
- Insulation R-value
- Thermal resistance of insulation. Hay/straw ≈ 2–3, wool blanket ≈ 3–4, sleeping bag ≈ 4–6.
- Room temperature
- Ambient air temperature where the hay box is kept.
What each result means
- Time above 75 °C
- How long food stays above 75 °C — the safe active cooking zone for stews and grains.
- Time above 60 °C
- Time before food drops into the danger zone (below 60 °C). Food should be eaten or reheated before this.
- Temperature after 2 hours
- Food temperature after 2 hours in the hay box.
- Temperature after 4 hours
- Food temperature after 4 hours in the hay box.
- Temperature after 8 hours
- Food temperature after 8 hours (e.g., overnight cooking).
- Fuel savings
- Estimated fuel/energy savings vs. conventional stove-top cooking for a stew.
- Insulation volume needed
- Volume of insulation material needed for 10 cm thickness around the pot.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersFood + liquid mass = 3, Pot diameter = 24, Starting temperature = 100, Insulation R-value = 3 = 5 input(s) provided
- Calculate Time above 75 °CTime above 75 °C = timeTo75Sec / 360017.7 = 17.7
- Calculate Time above 60 °CTime above 60 °C = timeTo60Sec / 360033 = 33
- Calculate Temperature after 2 hours97 = 97
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why doesn't the fuel savings percentage change when I adjust pot size or insulation?
Fuel savings is calculated from a fixed rule-of-thumb comparison — a 20-minute stovetop boil to bring the pot up to temperature versus a full 2-hour conventional stovetop simmer for a typical stew — and that comparison never references any of your entered values. Only the cooling-curve outputs (time above 75°C, time above 60°C, and the temperature-at-hour-marks) actually respond to changes in food mass, pot size, insulation R-value, or ambient temperature.
Why does more food mass keep the pot hot longer, even though a bigger mass also has more surface area to lose heat from?
In this pot geometry, the surface area is fixed by pot diameter alone, while thermal capacity scales with your food mass input (using ~3.9 kJ/kg·°C for food and water). Since the cooling time constant τ is thermal capacity divided by heat-loss rate, adding food mass without changing pot diameter increases τ directly, making the exponential cooldown slower and stretching out the time above both the 75°C and 60°C thresholds.
What's the practical difference between the '75°C' and '60°C' thresholds in the results?
75°C marks the safe zone for continued slow cooking — stews and grains keep cooking properly above this temperature. 60°C is the food-safety floor below which food shouldn't be left sitting, so the gap between the two hour figures tells you your safe window: eat, reheat, or finish the meal before the pot drops past 60°C.
Why would a smaller pot cool down faster even with the same food mass and insulation?
The model computes pot surface area from a cylindrical shape based on your entered pot diameter (with height fixed at 70% of diameter), and heat-loss rate is proportional to that surface area divided by insulation R-value. A smaller-diameter pot holding the same food mass has proportionally more surface area to lose heat through, shortening the time constant τ and cooling the food faster despite an identical thermal capacity.
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