“How much grit do I use?” is one of the most common questions we get. The answer is simpler than most people expect: measure by the weight of the rock, not by how full the barrel looks. Here’s the rule we use and how it works out for common barrel sizes.
The rule of thumb
In a rotary tumbler, use about 2 tablespoons of grit per pound of rock, for every step. That goes for the coarse grind, medium/fine, pre-polish and the final aluminum oxide polish.
- 1.5 lb barrel: about 3 tablespoons per step.
- 3 lb barrel: about 6 tablespoons per step.
- 6 lb barrel: about 12 tablespoons per step.
- 15 lb barrel: about 30 tablespoons per step.
Those amounts assume a barrel that is two-thirds to three-quarters full of rock, with water just below the top layer of stones. If you are short on rock, top up with ceramic or plastic spacers rather than running a half-empty barrel, and count the filler as part of the load.
Vibratory tumblers use less
A vibratory tumbler runs as a thick slurry rather than a barrel of water, so it needs much less grit. Plan on about 1 ounce of grit per step for a standard 3 to 5 lb load, with just enough water to make a creamy slurry. See our vibratory tumbler guide for the full schedule.
What happens when the amount is off
- Too little grit: stones grind against each other with nothing to cut, so they bruise and chip, and the step takes much longer.
- Too much grit: the slurry gets so thick the load slides as one lump instead of tumbling, and you waste grit without getting a faster result.
- Old grit: silicon carbide breaks down as it works. After a week it has done its job, so start each step, and each repeat of a step, with fresh grit.
Let the kit do the measuring
Our tumbler-specific refill kits are made for popular tumblers, including Lortone, Thumler’s, Harbor Freight, National Geographic and Smithsonian, so the grit is already matched to the barrel. For other tumblers, our standard grit kit is sized for a 3 lb rotary barrel, and bulk silicon carbide grit lets you measure out larger loads.
Not sure what your tumbler needs? Send us your tumbler model and barrel size and we’ll tell you exactly how much to use. For the full step-by-step schedule, read our four-step rotary guide.