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Sauna Stones Explained: How Much Stone, What Type, and Why It Matters

Sep 26,2026

The stones in a sauna heater are not decoration. They are the thermal store that turns a hot element into a soft, sustained steam — and getting them wrong changes how the sauna feels.

What the Stones Actually Do
A sauna heater has two jobs: heat the room, and produce steam on demand. The element does the first. The stones do the second.
Stone heats slowly and releases heat slowly. When you pour water over a hot stone, the water flashes to steam, and the stone's stored heat is what makes that steam immediate and sustained. An element alone would produce a sharp, short burst.
Stone mass is therefore why two heaters with the same power rating can feel completely different.
How Much Stone
Follow the manufacturer's stated capacity — it is set by the basket geometry and the element's thermal design, and overfilling or underfilling both hurt performance.
For reference, our 9 kW SAB-90H takes 18–22 kg of stone.
Underfilling reduces the thermal store, so steam is short and sharp. Overfilling packs the stones too tightly, restricts airflow past the element, and can cause the element to overheat.
What Kind of Stone
Sauna stones need three properties.
Heat tolerance. They must withstand repeated heating and sudden cooling from water without cracking. Stones that fracture shed fragments into the basket and can damage the element.
Even size. Stones of roughly consistent size leave voids between them for airflow. Mixed sizes settle and pack down, restricting flow.
Roundness. Smooth, rounded stones present fewer sharp edges. Sharp, angular stones are more prone to spalling.
Purpose-made sauna stones — typically olivine, peridotite or similar igneous rock — are selected for these properties. Decorative garden stone or river pebbles are not substitutes: they may contain moisture that turns to steam internally and fractures the stone.
Packing the Basket
Two rules matter more than any other:
Fill to the recommended weight, not to the brim. The basket should look full but the stones must not be pressed in.
Leave the element's airflow path clear. Follow the heater's instruction on how the stones should sit relative to the element, particularly around any guard or spacing cage.
Looking After the Stones
Rearranging. Stones settle with use, and small fragments accumulate at the bottom. Rearranging the stones periodically restores airflow. Do it when the heater is cold.
Removing fragments. Broken pieces reduce airflow and can jam against the element. Remove them.
Replacing. Stones degrade over time. Cracking, crumbling or a visible colour change through the stone indicates replacement is due. How often depends on use — a commercial sauna running daily will need stones replaced far sooner than a domestic one used weekly.
Fragments and water. Pour water over the stones, not into the basket cavity, and avoid pouring directly onto the element.
Stones and Sizing Together
Stone capacity and power rating work as a pair. A heater with an adequate element but an undersized basket will feel sharp and short on steam; a large basket with an inadequate element will not reach temperature.
This is one of the reasons to check stone capacity alongside kW when comparing sauna heaters — it is a performance figure, not an accessory detail.
Frequently Asked Questions
How often should sauna stones be replaced?
It depends on how often the sauna is used. Look for cracking, crumbling, or a colour change through the stone rather than working to a fixed interval. Commercial saunas used daily will need replacement far more often than domestic ones.
Can I use river rocks or garden stones?
No. Purpose-made sauna stones are selected for heat tolerance and low moisture content. Ordinary stones can contain moisture that fractures them when heated.
Do more stones mean a better steam?
More stone mass gives a longer, softer steam — up to the heater's stated capacity. Beyond that, overfilling restricts airflow and overheats the element.
Check Stone Capacity Before You Buy
If you are comparing sauna heaters, put the stated stone capacity next to the kW rating. Both figures describe performance.
Send us your room volume and we will confirm the rating and the corresponding stone capacity for your installation.

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