How many stones does a heater hold?

Your heater's manual tells you. That's the short answer, and no rule of thumb beats it. Harvia's advice is to check both the stone size and the amount in the manual that ships with the heater¹.

Where to look

• The printed manual that came with the heater.
• The maker's product page or downloadable manual for your exact model.
• The maker's stone page, if it lists weights per model.

Look for two numbers together: a weight (usually in kilograms) and a stone size. Harvia's rounded stone page, for example, says to check the right stone size for your heater from the heater manual².

Can't find the manual? Search the model name printed on the heater's rating plate, plus "manual". If you still can't find it, ask the maker.

What the numbers look like

They vary a lot. These are HUUM's figures, all marked "approximately" by the maker³:

That's one brand. Don't borrow a number from the table for a different heater. Your own manual is the one to trust.

Turning weight into boxes

Stones are sold by the box. HUUM's boxes are 15 kg for 5 to 10 cm stones³. Harvia's rounded 5 to 10 cm box is listed at 15 kg too². Box sizes differ between makers and stone types, so read the label.

Take the DROP, which needs approximately 55 kg of 5 to 10 cm stones³. Divide by the 15 kg box and you land between three and four boxes³. The manual's number is "approximately", so you may have stones left over. Don't force the extras in.

Stoves and other set-ups

Wood-burning stoves have their own numbers. HUUM says its HIVE Wood 13 stove requires approximately 90 kg of stones, 5 to 10 cm across³. Some set-ups use a separate stone holder too. For HUUM that's a stone cage, filled with approximately 30 kg of 5 to 10 cm stones³.

Wood-fired stoves also need the maker's manual and your local building and fire rules for flues, clearances and permits. They give off carbon monoxide, so never use one in an unvented room.

Too many, too few

Harvia warns that overloading a heater with too many stones can stop good airflow inside it and lead to overheating of the elements⁴. It also says too few stones won't give you the best sauna experience⁴.

Don't build a mound either. Harvia's loading tips say not to form a high pile of stones on top of the heater¹.

Once you've got the right amount, the loading page shows how to place it.