Mix water to 20 °C for film developing
Enter your hot and cold tap temperatures and the volume you need, and the calculator gives the mL of each. If the tap runs warmer than your target, switch to ice: it gives the grams of ice that bring warm water down to 20 °C (68 °F). The 38 °C (100.4 °F) preset is for color (C-41) processing.
How the amounts are worked out
Hot and cold tap water: the share of hot water is (target - cold) / (hot - cold). With a hot tap at 45 °C and a cold tap at 15 °C, a target of 20 °C is 5/30, so 167 mL hot and 833 mL cold make a liter. For C-41 at 38 °C with a 50 °C hot tap and a 15 °C cold tap, it is 657 mL hot and 343 mL cold. This is my arithmetic, and it counts 1 mL of water as 1 g.
Ice: the ice at 0 °C has to melt and then warm up to the target, and the tap water gives up that heat as it cools. Wikipedia's page on the enthalpy of fusion says that when melting 1 kg of ice at 0 °C, "333.55 kJ of energy is absorbed with no temperature change", and its worked example uses 4.18 J/(g⋅°C) for water. For a total volume V of water, the grams of ice are V x 4.18 x (tap - target) / (333.55 + 4.18 x tap). That formula is my arithmetic.
Worked example: 1000 mL at 20 °C from tap water at 26 °C needs about 57 g of ice and 943 mL of the warm water. Melting takes most of the heat: about 333.55 J per gram of ice, against 4.18 J for each degree of warming.
Ice needed to reach 20 °C
| Tap water | Ice for 500 mL | Ice for 1000 mL | Tap water to start with (1000 mL) |
|---|---|---|---|
| 22 °C (72 °F) | 10 g | 20 g | 980 mL |
| 24 °C (75 °F) | 19 g | 39 g | 962 mL |
| 26 °C (79 °F) | 28 g | 57 g | 943 mL |
| 28 °C (82 °F) | 37 g | 74 g | 926 mL |
| 30 °C (86 °F) | 46 g | 91 g | 909 mL |
What the numbers leave out
The sums assume the water trades heat only with itself. A room-temperature tank and reel soak up or give off heat too, and so does the air in the room. Rinse the tank and reel with water at your target first, then check the temperature with a thermometer after mixing. Ice straight from a freezer is colder than 0 °C, so it cools a little more than the calculator says.
Tap temperatures drift while the water runs, so measure them just before you mix. Kodak's XTOL sheet (J-109) says the water for mixing its powder should be at "normal room temperature, about 65 to 85°F (18 to 30°C)". That range is for mixing the stock solution. The development time depends on the temperature of the working solution in the tank.
When you cannot hit the target
If the water ends up a few degrees off, change the time to match. Measure the developer, then use the time and temperature calculator to move your 20 °C time to the temperature you have. It is fitted to Ilford's chart, which covers 18 to 27 °C. Keep the wash and fixer close to the developer temperature.
Color is stricter. CineStill recommends 102 °F (about 39 °C) for its Cs41 kit, and the Cs41 rolls page has its times. Use the temperature on your own kit's sheet. For the amounts of concentrate and water in a mix, the dilution calculator does the sum.
Sources
Heat of fusion and specific heat of water from Wikipedia, Enthalpy of fusion (opened and read for this page on 2026-10-18). Mixing water range from Kodak's J-109 (March 2008). The rest is my arithmetic. Nothing you enter is stored.