The kettle trick, quantified · Free calculator

Desired Water Temperature Calculator

Your DDT calculation says you need 780 g of water at 34 °C — now what? This calculator answers with kettle math: how much boiling water and how much regular water to mix to hit an exact amount at an exact temperature.

It’s the practical companion to the DDT Calculator: one tells you the temperature you need, the other tells you how to make it.

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? Desired Water Temperature Calculator

Opened from the Bread Recipe Supercalculator: the desired water amount is the recipe's mix water — its water ingredient rows, the water you actually temper and blend at the tap (the water inside the leaven and other wet ingredients counts toward hydration, but is not blended here) — and the desired water temperature is the one the DDT Calculator just computed for this recipe. Both are being supplied by the recipe on screen, and are highlighted in green below. The hot and regular water temperatures below remain yours to change.

? Calculate how much boiling water and regular water to mix to get a desired water amount at a desired temperature.

Parameter	Value
? Desired Water Amount
? Desired Water Temperature
? Boiling / Hot Water Temperature
? Regular Water Temperature
? Boiling / Hot Water to Use	--
? Regular Water to Use	--

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Desired Water Temperature

Why Blend Two Waters Instead of Heating One?

You rarely have a tap that delivers exactly 35°C on demand, and reheating water to a precise temperature is fiddly. It is far easier to combine water you already have — hot (kettle/boiling) and cool (tap or room-temperature) — in the right ratio. This calculator finds that ratio. Think of it as a weighted average: the target sits somewhere between your two source temperatures, and its position decides how much of each you pour.

The Formula

This assumes both waters share the same heat capacity and that heat lost to the bowl, air, thermometer, and spoon is small.

hotMass = totalMass × (targetTemp − regularTemp) ÷ (hotTemp − regularTemp) regularMass = totalMass − hotMass mixedTemp = (hotMass × hotTemp + regularMass × regularTemp) ÷ totalMass

Why Not Just Start With Boiling Water?

Two reasons. First, boiling water poured onto flour or a starter can kill yeast and lactic bacteria and prematurely gelatinise starch — you want warm dough, not scalded dough. Blending down to your true target protects the culture. Second, starting cool and nudging up with a measured shot of hot water is repeatable: you can hit 35°C the same way every bake, whereas "let the kettle cool for a bit" is guesswork.

Boiling Water Isn't Always 100°C / 212°F

At altitude, water boils cooler — roughly 0.5°C lower per 150 m of elevation — which is why the hot-water temperature is editable. A baker in Denver or the Andes enters their real boiling point instead of pretending every kitchen is at sea level.

Practical Baking Notes

Warm the measuring cup or bowl when precision matters; a cold vessel steals heat from your blend.
Measure water by weight, not volume, for repeatable dough-temperature work.
Stir the blended water before reading the final temperature.
If your target is outside the hot/regular range, the mix physically can't reach it — the calculator will warn you.
Pair this with the DDT calculator: DDT tells you what water temperature you need; this tells you how to make it.

Worked example

Your DDT calculation says you need 780 g of water at 34 °C, but the kettle is at 100 °C and the tap runs 18 °C. The mix fraction is (34 − 18) ÷ (100 − 18) = 19.5%: pour 152 g of boiling water, top up with 628 g of tap water, and you're within a fraction of a degree — no juggling a thermometer under a running faucet. Enter your own numbers above and the calculator does the algebra.

Where it fits in the bake

This is the hands-on partner to the mixing stage of the Baker's Process: the DDT Calculator decides what temperature your water should be, and this one tells you how to actually produce it with a kettle and a tap. Together they put dough temperature — the biggest lever on your fermentation clock — fully under your control.

Desired Water Temperature Calculator FAQ

How do I get water to an exact temperature without fiddling?

Mix boiling and tap water by weight. Because the math is linear, one calculation lands you within a degree on the first pour — that's precisely what this calculator computes from your target amount, target temperature, and the two source temperatures.

What temperature is my tap water?

It varies by season and region — often 5–15 °C in winter and 18–25 °C in summer — which is why the calculator asks instead of assuming. Run the tap for ten seconds and measure once; it stays stable enough to reuse all session.

Why does water temperature matter so much in bread?

Water carries more than twice the heat per gram of flour (specific heat ~4.18 vs ~1.8 J/g°C), so it dominates the final dough temperature — and dough temperature is the strongest single lever on how fast fermentation runs. Control the water and you control the clock.

Is this the same as the DDT calculator?

They're a pair. The DDT Calculator works out what temperature your mixing water must be to hit a target dough temperature; this one works out how to physically make water at that temperature. Answer first, method second.

Does the bowl or kettle temperature affect the result?

A cold ceramic bowl can steal a couple of degrees from a small amount of water, so for tight targets, mix in the vessel you measured with or rinse the bowl warm first. For typical bread quantities (500 g+ of water) the effect is small enough to ignore.

Want to know why this works? Every model and constant behind this calculator is explained, derived, and put to work in The Book of Sourjoe — the interactive book this calculator was built for.

These calculators are just the crust. Check out the Public Preview of the Sourjoe App — The Book of Sourjoe, recipes, and every baking tool in one place.

Questions? Comments? Send us a note at info@sourjoe.com.