What size should sauna stones be?

There's no single right size. Your heater's manual sets it. Harvia puts it plainly: always check the right heater stone size and amount from the manual included with the heater¹.

The sizes you'll see

Sizes are the width of the stone, in centimetres. Makers sell these ranges:

• 5 to 10 cm. Harvia says stones this size are usually used in electric heaters, because they fit well between the heating elements¹.
• 10 to 15 cm. Harvia says these larger stones are used in wood-burning heaters and in selected electric heaters with larger stone capacity¹.
• 3 to 5 cm. HUUM sells round stones in this range as well as 5 to 10 cm².

Models differ even within one maker. HUUM's CLIFF heater requires approximately 75 kg of stones 3 to 5 cm across². Its CORE requires approximately 40 kg of stones 5 to 10 cm across².

Why airflow matters

HUUM says heaters rely on unrestricted airflow to transfer heat from the heating elements to the stone bed³. When stone fragments and dust build up inside the heater, airflow becomes restricted³.

Mixing sizes in one heater

Many heaters take more than one size, with the bigger stones low and the smaller ones near the top.

• Sauna from Finland says that in wood-fired stoves, larger stones go at the bottom, and the higher up you fill, the smaller the stones should be⁴.
• It adds that small stones on top vaporise water better, because together they have a larger total surface area than large stones⁴.
• Harvia says smaller stones are also used on the top layer of wood-burning heaters, since they create more surfaces for steam¹.

Surface area is just how much stone water can touch. More touching means more steam from the same splash.

Rounded stones are a special case

Rounded stones change the picture a little. Harvia says that if you use larger rounded stones of 10 to 15 cm, you should finish the top layer with smaller 5 to 10 cm stones¹. That gives the smallest stones the top spot¹.

HUUM suggests a denser top layer made with slightly smaller stones². Its page also says you'll want a mix of smaller and larger stones so the heater fills evenly without noticeable gaps².

When the stones shrink

Stones break down with use, so the size you loaded isn't the size you'll have in a year. Harvia says stones disintegrate slowly but gradually because of the temperature swings¹. When a stone has broken into small pieces, it no longer counts as the size your manual asked for. The replacement page covers what to look for.

Next step

Once you know the size, work out how much to buy. The weight page shows where to find that number.