How many kg of stones for your heater — by model

The amount of stone your heater needs is set by the heater, not by a rule of thumb. A small wall kiuas (sauna stove or heater) takes a single 20 kg box. A tall open basket can swallow ten times that. Buy too little and you get a weak, short-lived löyly (the burst of steam from water on hot stones). Pack in too much and you choke the airflow the element needs to stay cool. This guide gives the real numbers by heater class and by named model. It then covers stone type, correct size, and how to load them safely.

Why stone weight tracks heater type, not just kilowatts

A heater's stone capacity is a design number, not something you calculate from its power rating. Two 9 kW heaters can hold wildly different loads. A slim wall unit sizes its rock cavity for a small bed of stones near the elements. A pillar or basket heater surrounds a tall element column with rock, so it stores far more heat between water throws.

That stored heat is the point. More stone mass means the room recovers faster after each ladle. The steam stays soft and round instead of sharp and thin. It is why big-basket heaters feel gentler even at the same wattage. The trade-off is heat-up time and floor space, since all that rock takes longer to warm and needs a wider footprint. A 250 kg basket can want an hour to reach temperature, while a 20 kg wall heater is ready in half that.

So the correct figure is always the one printed on your model's spec sheet, not a generic "2 to 3 kg per kilowatt" estimate. Use the ranges below to sanity-check what you find, then order the exact weight the manufacturer lists. When a spec gives a range, fill to the middle of it and leave the top layer loose.

Stone load by heater class and model

The table groups the common residential heater styles with real figures pulled from manufacturer specs. Weights are approximate because stone sizes vary, but they are close enough to order by.

Heater class	Typical models	Output	Stone load
Small wall	Harvia Vega BC60, Tylö compact	4.5–6 kW	16–25 kg
Mid / large wall	Harvia Vega BC80/BC90	8–9 kW	25–30 kg
Pillar	Harvia Cilindro PC70–PC110	6.8–11 kW	60–120 kg
Open basket	HUUM Drop 55 kg, Cliff 75 kg, Hive 250 kg	6–18 kW	55–250 kg

A few model notes fill in the picture. Small wall heaters live at the light end: a 6 kW Harvia Vega holds roughly 16–20 kg, and a compact Tylö takes about 20 kg of vulcanite. The Harvia Cilindro pillar range climbs steeply, from around 60 kg on the smaller PC70 up to about 120 kg on the 11 kW PC110. HUUM shows the widest spread of any brand. The floor-standing Drop takes about 55 kg, the slim Cliff about 75 kg of smaller stones, and the HUUM Hive basket holds a full 250 kg. Combi units that also make steam, like the Tylö Sense Combi, sit at the light end near 20 kg because the steam tank does part of the work.

Wood-burning stoves run their own scale entirely. A small IKI basket carries around 120 kg, mid models 160–240 kg, and the largest sauna stoves top 400 kg. If you own a wood burner, the manual's stone weight is not optional guidance. Underfilling a firebox-style heater lets flame lick bare metal and warps the stove.

Which stones to buy, and which to never use

Get the material right before you weigh anything. Three stone types cover the home market. Olivine diabase is the Finnish default, dark and dense with very low water absorption. Peridotite is a denser premium cousin with slightly better heat retention. Vulcanite is a natural volcanic rock that many heater brands supply and test with. All three shrug off the thermal shock of a cold water splash landing on a 400 °C (752 °F) surface.

Stone type is also a heater rule, not just a preference. Some combi heaters forbid ceramic stones outright, because ceramic can overheat and destroy the resistors and void the warranty. When a manual names an approved stone, treat that as a hard constraint. If in doubt, buy the brand's own boxed stones, which come pre-sorted in the right material and size.

Stone size and how to load the heater

Size matters as much as weight. Most heaters want stones 5–10 cm across, large enough to leave air channels between them. Some slim heaters with an internal air tunnel, like the HUUM Cliff, use smaller 3–5 cm stones instead. Big open baskets can take the larger end of the range comfortably. Match your model's stated size band so the load nests without bridging.

Loading is a loose, deliberate job, not a pour. Work through it in order:

1. Rinse natural stones briefly under cold water to wash off transport dust, then let them drain.
2. Place the largest stones on the bottom and against the sides, around and between the heating elements.
3. Fill the middle and top with the smaller stones, standing elongated ones on end so heat can climb through the load.
4. Leave air gaps everywhere. Never wedge a stone against an element or press the bed down to fit more in.
5. Stop at the rim. Do not heap stones above the top edge or cover the grille on a tower heater.

The reason for all the air is simple. The elements shed heat into moving air, and a packed bed traps that heat against the metal, which runs the element hot and shortens its life. A loose load also throws a better löyly, because water can reach hot rock deep in the pile instead of sheeting off a solid crust.

Refreshing and topping up the load

Stones are wear parts, so the weight you loaded is not the weight you keep forever. Every session cycles them from room temperature to several hundred degrees and back, and each splash cracks them a little more. As they break down, they slump and pack tighter, and the gaps you built in start to close.

Inspect the load once a year at minimum. Lift the top layer and pull out any stone that crumbles or splits in your hand. Brush the dust and chips out of the base while you are in there. For weekly home use, plan a full stone change every one to two years. For daily or commercial use, expect to restack every few months and replace the whole load yearly. A cheap kitchen scale settles any doubt about how much you have pulled and how much to add back. When you refresh, weigh the new stones back up to the model's spec so you never drift below the correct load. The full swap-out routine, with the signs a load is finished, lives in our guide on when and how to replace sauna stones. While the heater is stripped, it is also the natural moment to run your floor and drain cleaning pass.

One safety habit ties the whole job together: let the heater cool fully before you touch the stones. Switch it off, trip the breaker, and wait several hours or overnight. Rearranging a hot load over a live element is the one real hazard here, and it is entirely avoidable. Get the weight, the material, and the loading right, and the stones become the part of the sauna you almost never think about.