Sauna Rocks: Best Stones, How Many and When to Replace
What do sauna rocks actually do?
Sauna stones are a heat store and a steam generator. In an electric or wood-burning sauna stove with rocks, the heating elements or fire heat the stones, and the stones heat the air by convection and radiation. When you ladle water onto them, they flash it into steam.
That burst of steam is called löyly in Finnish. A traditional sauna normally runs at about 10-20% relative humidity, according to a 2018 review in Mayo Clinic Proceedings, and a ladle of water raises humidity briefly so the room feels hotter. A heater with too few stones, or the wrong stones, produces harsh, sputtering steam that disappears quickly.
Infrared saunas do not use rocks. They heat with radiant panels and are designed to run dry. If you want a hot stone sauna with steam, you need a traditional stove; see our sauna heater guide.
What are the best sauna stones?
Good sauna stones share four properties: high density, low porosity, resistance to thermal shock, and no harmful odor or dust when heated. Most of the best options are igneous rocks, which form when molten rock cools and solidifies. The U.S. Geological Survey classifies peridotite and gabbro as intrusive igneous rocks, which cool slowly underground and form coarse, interlocking crystals, and basalt as an extrusive rock that cools quickly at the surface.
| Stone type | Rock family | Strengths | Watch-outs | Verdict |
|---|---|---|---|---|
| Peridotite | Intrusive igneous | Very dense, stores heat well, soft steam | Heavier per volume | Excellent |
| Olivine diabase | Igneous | Durable, widely used in heaters | Check for cracks yearly | Excellent |
| Gabbro | Intrusive igneous | Dense, strong | Can be hard to find in heater sizes | Very good |
| Basalt | Extrusive igneous | Dense, affordable | Vesicular (bubbly) pieces are weaker | Good |
| Soapstone | Metamorphic | High heat capacity, gentle steam | Softer; can be costly | Very good |
| Granite | Intrusive igneous | Common, cheap | Quartz content can crack and spall | Fair, not ideal |
| Lava rock | Extrusive, porous | Light | Porous, crumbles, holds water | Avoid unless specified |
| River rock or sandstone | Often sedimentary | Free | Porous, may crack or burst | Avoid |
| Ceramic heater stones | Manufactured | Uniform shape | Only for heaters designed for them | Follow manual |
Split or rounded sauna rocks?
Split (rough, angular) stones have more surface area, so they heat and vaporize water faster and give softer steam. Rounded, tumbled stones look neat and allow good airflow, but release steam less evenly. Many users mix them: split stones for the bulk of the load and rounded stones on top for appearance.
Can river rock be used as sauna stones?
River rock is a common question, and the short answer is no. Stones from a riverbed or garden are often sedimentary or a mix of unknown types. The USGS describes sedimentary rocks such as sandstone, siltstone, limestone and conglomerate as compacted, cemented deposits of fragments, which can hold water in their pores.
When a porous, water-logged stone is heated quickly, trapped moisture turns to steam and can crack the stone or throw off fragments. Limestone can also break down at high temperatures, and unknown stones can release odors. Rocks sold for saunas are selected and sized for the job, so they are the safer choice.
How many sauna rocks do you need?
Always use the weight and size range in your heater's installation manual. Electric sauna heaters are tested to safety standards such as IEC 60335-2-53 and, in North America, UL 875, which California's mechanical code requires along with installation to the manufacturer's instructions. The stone load is part of that tested design.
Typical ranges for planning:
| Heater type and size | Typical stone load | Typical stone size |
|---|---|---|
| Small electric, 3-4.5 kW | 15-25 kg / 35-55 lb | 2-3 in / 5-8 cm |
| Mid-size electric, 6-9 kW | 25-50 kg / 55-110 lb | 2-4 in / 5-10 cm |
| Large or commercial electric | 50-120+ kg / 110-265+ lb | 3-4 in / 8-10 cm |
| Wood-burning stove | 40-150+ kg / 90-330+ lb | 3-6 in / 8-15 cm |
Too few stones expose the elements, which overheat and give dry, sharp heat. Too many, or packed too tightly, block airflow, slow heat-up and can trip the high-limit cutout.
How to load sauna rocks, step by step
- Turn off power at the breaker and let the heater cool completely.
- Rinse new stones in a bucket to remove dust, then let them dry.
- Sort by size. Set aside cracked or flaking pieces.
- Place larger stones at the bottom and around the elements, standing some on edge.
- Stack loosely. Leave gaps so air can flow upward between stones.
- Keep elements covered but never wedge stones against them or bend them.
- Top with medium stones, so water spreads over many hot surfaces.
- Run a first heat cycle with good ventilation. A faint smell the first time is normal as dust burns off.
Pouring water on sauna rocks: how to do it well
- Use a wooden ladle and pour 1-2 small ladlefuls at a time, spreading them over the stones.
- Use clean tap water. Avoid seawater, pool water or hard well water heavy in minerals, which leave deposits and can corrode elements.
- If you like aromas, add a few drops of sauna-specific scent to the water bucket; never pour concentrated oils directly onto hot stones.
- Do not throw water on an electric heater that is not designed for it, or on the control unit.
- Wait for the steam to settle before adding more. Repeated large splashes cool the stones and produce less steam.
For more on how steam changes the feel of a sauna, see our sauna temperature guide.
When should you replace sauna stones?
Heat cycles and water slowly break stones down. Signs it is time to rearrange or replace:
- Cracked, crumbling or flaking stones
- Grit or sand in the bottom of the heater
- Longer heat-up times than when the stones were new
- Steam that hisses and vanishes rather than rolling down softly
- Stones settling so the elements are exposed
As a rule of thumb, sort and top up stones once a year in a home sauna used 3-4 times a week. In gyms, hotels and spas that run daily, check every 3-6 months. When restacking, vacuum out the heater base and discard anything smaller than about 1 in (2.5 cm). Our how to clean a sauna guide covers the rest of routine maintenance.
Buying sauna stones for sale: what to check
When comparing sauna rocks for sale, look for:
- Rock type stated clearly (peridotite, olivine diabase, basalt, soapstone)
- Size range that matches your heater manual
- Pack weight, so you can calculate how many boxes you need
- Pre-washed stones to cut down on dust
- Consistent shape, split or rounded, as you prefer
For add-ons, browse our sauna accessories range. Mars Sauna traditional stove saunas use electric heaters of roughly 3.5-6 kW, so check each model's manual for its stone load. Distributors stocking traditional saunas can request a quote.
Key takeaways
- The best sauna rocks are dense igneous stones such as peridotite, olivine diabase, gabbro and basalt, or soapstone, about 2-4 in (5-10 cm) across.
- Avoid river rock, sandstone, limestone and porous lava rock, which can crack or crumble when heated.
- Load the weight specified in the heater manual, large stones at the bottom, stacked loosely for airflow.
- Pour small amounts of clean water to create soft löyly; never pour oils directly on the stones.
- Sort and replace stones about once a year at home and every 3-6 months in commercial saunas.
Frequently asked questions
What are the best rocks for a sauna?
The best sauna rocks are dense, non-porous igneous stones such as peridotite, olivine diabase, gabbro and basalt, plus soapstone. They store heat well, tolerate repeated heating and splashing, and resist cracking. Choose stones about 2-4 in (5-10 cm) across, and always follow the size and weight your heater's manufacturer specifies.
Can river rock be used as sauna stones?
River rock is not recommended for sauna stones. Many river stones are sedimentary or mixed types such as sandstone, limestone or quartz-rich pebbles, which can be porous, hold water and crack or spall when heated rapidly. Unknown rocks may also release odors or dust. Buy stones sold specifically for sauna heaters instead.
How many sauna rocks do I need?
The number of sauna rocks depends on the heater, so check its manual for the specified weight. As a rough guide, small home electric heaters of 3-6 kW often take about 20-40 kg (45-90 lb) of stones, while larger electric and wood-burning stoves can take 40-100 kg (90-220 lb) or more.
How often should you replace sauna rocks?
Replace or sort sauna rocks about once a year in a home sauna used a few times a week, and every few months in a busy commercial sauna. Remove the stones, discard any that are cracked, crumbling or flaking, rinse the rest, clean out grit from the heater and restack with fresh stones as needed.
What happens when you pour water on sauna rocks?
Pouring water on hot sauna rocks turns it into a burst of steam, known in Finnish as löyly. Relative humidity rises briefly from about 10-20% and the room feels noticeably hotter for a minute or two, even though the thermometer may barely move. Use small amounts of clean water and only on heaters designed for it.
Can you use lava rocks in a sauna?
Lava rock is generally a poor choice for a traditional sauna heater unless the manufacturer specifies it. Volcanic lava rock is very porous and light, so it stores less heat per piece, can crumble into grit that clogs the heater, and absorbs water. Dense igneous stones such as olivine diabase or basalt perform better.
Sources
- U.S. Geological Survey: What are igneous rocks?
- U.S. Geological Survey: What are sedimentary rocks?
- IEC 60335-2-53: Particular requirements for sauna heating appliances and infrared cabins
- California Mechanical Code 939.1: Electric sauna heaters (UL 875)
- Laukkanen et al. (2018) Cardiovascular and Other Health Benefits of Sauna Bathing: A Review of the Evidence. Mayo Clinic Proceedings
