Milk Kefir Calculator: How Many Grains for Your Milk

By Nik, founder of 東歐嚴選｜kefir.link and former chef · First published 20 October 2024 · Updated 3 November 2026

Your kefir came out runny like milk, or it turned sour and split overnight? Most of the time the grains are fine and only the amount of milk does not suit the temperature in your kitchen. Enter your room temperature, your milk and your grains, and the calculator tells you how many grams of grains to use, or how much milk your grains can handle.

Work out your amounts

Choose “Grains I need” if you know how much milk you want to ferment. Choose “Milk I need” if you have a set amount of grains. Then move the sliders to match your kitchen.

How many grains for 1 litre of milk?

These are the calculator’s results for 1 litre of fresh milk, active grains and 24 hours, with the same temperature day and night. Warmer milk at the start needs fewer grains than milk straight from the fridge.

Have 10 g of grains? At 24 °C with room-temperature milk, that is enough for about 430 ml of milk. Sleepy grains need 20% more.

How the calculator works

• A reference batch: it starts from 21.43 g of active grains per litre of milk, fermented for 20 hours at 28 °C.
• Room temperature: every degree cooler than 28 °C adds about 1.5 hours of work for the grains, and every degree warmer takes it away. The calculator uses the average of your day and night temperatures.
• Cold milk: milk from the fridge slows the first hours. The calculator counts this for the first 6 hours, and the effect gets smaller the longer you ferment.
• Your time: a longer fermentation needs fewer grains, a shorter one needs more.
• Sleepy grains: grains that are still recovering get 20% more.

The temperature rule follows published research: in a 2022 study on milk kefir, batches at 25 °C turned sour faster than batches at 20 °C. Every jar is still a little different, because grains, milk and kitchens differ, so treat the result as a good starting point and adjust after tasting.

1. Room temperature matters most

• The calculator covers 16–32 °C. That is the range in which milk kefir ferments well at home.
• Summer: above 30 °C the grains work very fast. Keep the jar in the coolest spot at home, out of the sun, and strain earlier if it is already thick.
• Never heat it: yogurt makers, rice cookers, microwaves and ovens kill the grains. Room temperature is all they need.
• Winter: an 18 °C room works too. Use more grains or a longer time, up to 36 hours.
• Not in the fridge: the fridge is for resting grains, not for making kefir. Below about 16 °C, fermentation becomes very slow and the taste can turn unpleasant.

2. Milk temperature at the start

Fresh milk from the fridge is about 5 °C. It takes several hours to reach room temperature, and the grains work slowly until it does. You have two easy choices:

• Let the milk warm up: leave it at room temperature for 1–3 hours before you add the grains, especially in cold weather.
• Or add a few more grains: set the milk slider to the temperature of your milk, and the calculator adds the grains it needs.

3. Active or sleepy grains

• Active: grains you use every day or two in fresh milk. They ferment at their normal speed.
• Sleepy: grains that just arrived by post, or that rested in the fridge. They work more slowly for a while, so the calculator adds 20%.
• The first 3 batches after a rest: keep “Sleepy” selected. After that the grains are back to normal, and you can use the calculator with “Active”.

1. Put the grains in fresh milk
2. Leave it at room temperature for about 24 hours
3. Strain out the grains
4. Kefir goes in the fridge; the grains go into fresh milk

4. How long to ferment

• Start with 24 hours. It is the easiest rhythm: one batch a day, at the same time.
• The calculator allows 20–36 hours. A shorter time gives milder kefir; a longer time gives tangier, thicker kefir.
• Never longer than 36 hours in all. If it still looks like milk by then, see when it goes wrong.

How to tell your kefir is ready

• Not ready yet (left): it still pours like milk. Leave it longer, but no more than 36 hours in all.
• Just right (middle): evenly thick, with a clean sour smell and maybe a little clear whey. This is what the calculator aims for.
• Fermented too long (right): thick curds on top and yellow whey underneath. It is still kefir, only sourer. Next time, strain earlier or use more milk.
• The pH strip: shake the jar, put one drop on the strip. Dark blue turns green when the batch is just right (about pH 4) and yellow when it fermented longer. Anything from pH 3.0 to 5.0 is a success.

How pH changes during fermentation

Fresh milk has a pH of about 6.5–6.7. The bacteria and yeasts in the grains turn lactose into lactic acid, so the pH drops and the milk becomes tangy and thicker. Around pH 4.6 the milk proteins start to set into curds.

You do not need to test every batch. Once you know how your kefir looks and smells when it is right, your eyes and nose are enough. If you have never tasted kefir, try a bottle of our ready-made milk kefir first, so you know the taste you are aiming for.

When something does not look right

Thin, as if it failed

Test it first: shake the jar and put one drop on a pH strip. If the colour changes, it worked and only the ratio needs adjusting. Grains that just arrived or came out of the fridge make thinner kefir for the first 3 batches.

Cold weather, still milk after 24 hours

If the strip has not changed, leave the cover on and keep fermenting, no longer than 36 hours in all. Still no change? Pour it away. Next time, choose “Sleepy” in the calculator and let the milk warm up for 1–3 hours first.

Too sour, or split into curds and whey

The grains had too little milk for the warmth of the room. Next time, use more milk, or strain a few hours earlier. You can still drink this batch: stir the whey back in or strain it off.

Pink, green, red, blue or black spots

This may be mould. Throw away all the grains and kefir, even the parts that look fine, and send us a photo on LINE.

Frequently asked questions

Can I use this calculator for water kefir?

No. It is made for milk kefir grains in dairy milk. Water kefir grains ferment sugar water and need different amounts.

Can I use soy milk or goat’s milk?

Goat’s milk works like cow’s milk. Soy milk ferments in about 18 hours at about 1:50 in summer and 1:30 in winter; judge it by thickness, not with a strip. After every six days of plant milk, give the grains one day of fresh milk.

My grains have grown. What do I do with the extra?

Weigh them and use “Milk I need” to see how much milk they can handle now. You can also make a bigger batch, or rest some grains in the fridge in fresh milk for up to a month.

Why are day and night temperatures separate?

A batch sits for a whole day and night, and many homes cool down at night. The calculator uses the average of the two.

I am going away. What happens to my grains?

Put them in a glass jar, cover them with fresh milk, close the lid and put them in the fridge for up to a month. When you are back, choose “Sleepy” and use less milk for the first 3 batches.

Does the calculator tell me if my kefir is safe?

No. It only works out the amounts. Use fresh milk and clean tools, and throw the batch away if you see mould or it smells rotten.

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