Cooking Time Calculator
Estimates roasting time using the two-thirds power law that actually governs heat transfer, instead of the minutes-per-pound rule that cannot be right at both ends of the scale.
Cooking Time Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- A scaling law needs comparable geometry; it cannot compare a spatchcocked bird with a whole one.
- Oven calibration, starting temperature and door-opening habits move real times by more than the arithmetic.
- Not applicable to thin cuts, moist-heat cooking or barbecue, where other mechanisms dominate.
- Estimates never establish doneness; only a thermometer does.
In short: Cooking time does not scale with weight. Heat enters through the surface but has to warm the volume, so time rises with mass to the two-thirds power: doubling the weight multiplies the time by 1.59, not 2. That is why published charts quietly drop from “20 minutes per pound” on a small bird to “15” on a large one — the folk rules already contain the power law, they just never admit it.
Formula
time ∝ mass2/3
t = tref × (W ÷ Wref)2/3
Panofsky: t (hours) = W2/3 ÷ 1.5 at 325°F
doubling the weight → ×22/3 = ×1.587
[('2/3', 'Surface area grows as the square of size, volume as the cube, so time goes as mass to the two-thirds.'), ('Wref', 'The weight the published chart rate was written for. The estimate is exact there and scales away from it.'), ('1.587', 'The factor for doubling the weight. Not 2.'), ('±15%', 'The honest width of any time estimate before a thermometer is involved.')]
Worked Example
- Weigh the piece, not the tray. The calculation is about one object and the distance to its centre.
- Pick the cut so the reference rate is right. Each cut has its own chart rate and usual oven temperature.
- Let the power law scale it. Time moves with the two-thirds power of the weight ratio.
- Treat several pieces as parallel. Separate pieces do not add their weights together.
- Finish on the thermometer. The clock plans the day; the probe decides doneness.
A 12 lb unstuffed turkey at the standard 15 minutes per pound takes about 3 hours. Scale to 20 lb and the power law gives 4 h 13 min, while multiplying 15 by 20 would have claimed 5 hours — 47 minutes of unnecessary overcooking. Run it the other way and an 8 lb bird needs about 2 h 17 min rather than the 2 hours the linear rule suggests.
Strengths & Limits Of This Model
Where this engine is strong
- Uses the heat-transfer relation instead of a rule that cannot hold
- Reproduces published chart rates at their reference weight
- Shows what the linear rule would have got wrong, and by how much
- Treats multiple pieces as parallel rather than additive
Where it stops
- Requires knowing the cut to pick a sensible reference rate
- Cannot account for shape changes such as spatchcocking
- Still an estimate; the thermometer is the authority
Practical Use Cases
Planning a roast around a serving time
Gives a defensible estimate and an honest range rather than false precision.
Scaling up a recipe that worked at a smaller size
Your own measured time plus the power law beats any published chart.
Cooking several joints at once
Shows that parallel pieces do not add their weights together.
Understanding why two charts disagree
They were written at different reference weights, and both are linear approximations to the same curve.
Methodology & Editorial Standards
Cooking time is computed from the heat-transfer scaling relation rather than from a linear minutes-per-pound rule. Heat enters a roast through its surface, which scales with the square of linear size, while the energy required scales with the volume, which goes as the cube; mass also goes as the cube, so time scales as mass to the two-thirds power. This is the relation McGee sets out in On Food and Cooking and the one Panofsky reduced to t = W to the two-thirds over 1.5 hours at 325°F. Each cut carries a published chart rate in minutes per pound together with the reference weight that rate was written for; the calculation converts that pair into a total at the reference weight and then scales it by the two-thirds power of the weight ratio, so it reproduces the published figure exactly at the reference weight and diverges from the linear rule in both directions away from it. Validation against an independently published heat-transfer table which quotes 1, 1.6, 2.6 and 4.1 hours for 2, 4, 8 and 16 lb roasts returns 1.0, 1.6, 2.5 and 4.0 hours. Multiple pieces are treated as cooking in parallel rather than in series, since conduction into one piece is independent of conduction into another, and a fifteen percent planning band is reported because oven calibration alone commonly accounts for that much variance. Every result is subordinated to the USDA safe minimum internal temperatures, which the page states rather than implies.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Cooking Time Calculator — 20 Expert FAQs
20 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why is minutes per pound wrong?
Because it assumes cooking time rises in proportion to weight, and it does not. Heat enters through the surface, which grows with the square of size, but has to warm the volume, which grows with the cube. The result is that time follows mass to the two-thirds power. A rule of so many minutes per pound is exact only at the weight it was written for and drifts in both directions away from it.
How much longer does a bird twice the size take?
About 59% longer, not 100%. The factor is two to the two-thirds power, which is 1.587. So a 10 lb bird taking 2 hours implies a 20 lb bird takes roughly 3 hours 10 minutes, not 4 hours. This is the single most useful consequence of the power law and the one that saves the most overcooked dinners.
Why do charts say 20 minutes per pound for small birds and 15 for large?
Because they are describing a curve with two straight lines. If time really were proportional to weight, one figure would serve every size. The fact that every published chart quietly reduces the rate as the bird gets bigger is direct evidence for the two-thirds power law — the traditional advice already encodes the physics, it simply never explains why.
If I cook two roasts at once, do I double the time?
No. Separate pieces cook in parallel, because the heat travelling into one has nothing to do with the heat travelling into the other. Two 2 lb loins each take the time of a 2 lb loin. Allow perhaps five to ten percent extra because the additional cold mass pulls the oven temperature down and it takes longer to recover, and rotate the pans once, but do not add the weights.
Does the shape of the meat matter more than the weight?
Yes, strictly speaking. What governs the time is how far heat has to travel to the centre, and weight is only a proxy for that. A spatchcocked chicken cooks far faster than a whole one of identical weight because flattening it halves the distance to the centre. A rolled joint is thicker than the same meat laid flat. Use the power law to scale within a shape, not to compare across shapes.
What is the Panofsky turkey formula?
Pief Panofsky, then director of the Stanford Linear Accelerator Center, objected to the linear minutes-per-pound rule and derived t = W to the two-thirds power divided by 1.5, giving hours for a stuffed turkey in pounds at 325°F. The constant 1.5 was fitted empirically. It gives about 2 h 40 min for an 8 lb bird and 5 h 33 min for a 24 lb one, which tracks the published charts closely.
Should I trust the timer or the thermometer?
The thermometer, always. Time is for planning your day; internal temperature is what establishes both safety and doneness. Ovens are commonly out by 25°F or more, birds differ in shape, and starting temperature varies. An instant-read thermometer is the cheapest meaningful upgrade available to a home cook, and it makes every timing estimate on this page merely a convenience.
Does a bone-in roast take longer than boneless?
Usually yes, at the same weight, though less than people expect and for two competing reasons. Bone conducts heat poorly compared with metal but the geometry matters more: a bone-in joint is generally thicker through the middle than the same weight boned and laid out. Bone also throws off thermometer readings badly, so always probe the meat well away from it.
How accurate is any cooking time estimate?
Plan on plus or minus fifteen percent even from a good calculation. Oven calibration alone routinely accounts for that much, and starting temperature, shape, pan material, how often the door is opened and whether the joint was properly rested at room temperature all add to it. That is not a weakness of the arithmetic; it is the real spread, and any tool quoting a single exact minute figure is overselling.
Does the two-thirds rule work for everything?
It works well for solid pieces cooked by dry heat where the shape stays roughly similar as the size changes — roasts, whole birds, large joints. It breaks down for thin items where the surface dominates, for anything cooked in liquid where convection changes the picture, and across genuinely different shapes. It is a scaling law, and like all scaling laws it needs the geometry to stay comparable.
Is this cooking time calculator free to use?
Yes. It is free, requires no account, and has no usage limits. ApexConverter is funded by contextual advertising, never by selling user data.
Is my data sent to a server?
No. The engine runs as Vanilla JavaScript inside your browser under our Zero-Server Client-Side Execution model. Your figures are computed locally and are never transmitted, logged, or stored.
How accurate is this calculator?
It applies the standard closed-form formula in IEEE-754 double precision, rounding only at the display layer. The engine is reconciled against an independent reference implementation before release.
Does it work on mobile?
Yes. The interface is mobile-first with numeric keypad hints and is tested down to a 320-pixel viewport with no horizontal scrolling.
Can I use it offline?
Largely, yes. Because computation is client-side, the page continues to calculate without a network connection once it has loaded.
Which currency does it use?
Amounts display in US$ accounting format, but the underlying mathematics is currency-agnostic. The result is identical in any currency, so simply read the figures in your own.
Why does a result show an em-dash?
An em-dash indicates the calculation is not defined for the inputs given — typically a division by zero or a value outside the valid domain. We show a dash rather than a misleading number.
How do I report an error?
Email apexconverter.praxiscalc@gmail.com with the tool URL, your exact inputs, the output received and the output you expected. Verified mathematical errors are patched within 72 hours.
Where can I find related cooking time calculator tools? (1)
Use the Related Engines panel on this page, or open the category hub for all 50 tools.
Where can I find related cooking time calculator tools? (2)
Use the Related Engines panel on this page, or open the category hub for all 50 tools.