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Our Clay Hole Calculators
Shrinkage. How much clay to start with. What a piece will hold. Firing costs. What to charge for it. The hours between you and your next close friend. Shelf space. Drying time. Bag math. Cone temperatures. Every calculator has its own controls, so pick the one you need and let this page do the arithmetic while your hands stay muddy.
Calculator no. 1
The shrinkage calculator
Clay shrinks as it dries, then shrinks again in the glaze fire, and every clay we stock shrinks a different amount. Pick the bag you are throwing from and the math switches to its maker’s number. B-Mix gives up about 12% from wet to done.
Throw it at 10″ today and it comes out of the glaze kiln at about 8.8″.
Three sizes people actually chase, in B-Mix:
Berlin Gray’s maker lists drying shrinkage (5.7%) and firing shrinkage (6.7% at 1300°C) separately. We combined them to about 12%. Their test firing runs hotter than ours, so treat that number as the high end.
Shrinkage numbers are the manufacturers’ averages. Clay is a natural material and every batch has a personality, so give yourself a little grace in either direction.
Calculator no. 2
The clay weight calculator
How much clay to wedge up before you sit down. Pick what you are making, be honest about your throwing, set the size you want it to end up, and it hands you a starting weight. Too little clay and the piece comes up short. Too much and you fight it the whole way.
About 567 grams for a mug that ends up 3.5″ wide and 4″ tall. On the wheel that is roughly 3.98″ by 4.55″, because it shrinks.
Add about a quarter pound for the handle. It gets pulled from a separate lump, and it is the part beginners run short on.
Weights round to the nearest quarter pound and assume about 12% total shrinkage, which is most of our clays. Porcelain shrinks a little more and 52 Buff a little less, and neither moves the answer much. Standard sizes are pre-filled for each form; change them to yours. Above ten pounds or so, most potters throw in two sections and join them.
Calculator no. 3
The volume calculator
How much a piece holds, from a tape measure instead of a jug of water. It works on anything round: measure the inside depth and the inside width at a few heights, and the math stacks the slices. If the piece is not fired yet, it also tells you what it will hold once the kiln has had its say.
Measured to the rim, that is 31.65 cubic inches, 519 mL, or about 2.2 cups.
Capacity shrinks by the cube. At 12%, a piece loses about 32% of its volume between wet and fired. That is why a mug that held 17.5 ounces of water on the wheel pours 12 when it is done, and why the “fits a can of soda” test has to wait for the kiln.
No tape that fits inside? Wrap a soft tape around the outside, divide by 3.14, and take off two wall thicknesses. For a bowl, the three readings are the foot, halfway up the curve, and the rim. A piece that leans, wobbles or squares off will read a little high.
Bone dry and bisqued pieces have already done part of their shrinking. We estimate that a little over half of the total shrink happens in the glaze fire, which matches the one clay whose maker splits the number, so treat the fired estimate as close rather than exact.
Calculator no. 4
The firing cost calculator
Glaze firings cost five cents per cubic inch of the box your piece fits inside. Wide, times long, times tall. Bisque firings are free. That’s the entire pricing model, and here it is with sliders.
Under 125 cubic inches, so the flat $4.80 minimum applies. Told you it was friendly.
Okay, that one is beyond everyday math. Firing charges cap at $600, and the kiln itself tops out around 22″ wide by 44″ long by about 52″ tall. Building something enormous? Wonderful. Talk to us at the studio first and we’ll figure it out together.
Anything 125 cubic inches or under fires for a flat $4.80. A mug, basically.
Small pieces that fit on one 6″ cookie, with a quarter inch between them, count as one piece when they total under 125 cubic inches. One cookie, one $4.80.
Bisque firings cost members nothing. Zero. We only charge for glaze firings.
Charges cap at $600, and truly giant pieces deserve a real conversation. Talk to us before you build the monument.
The firing page charges in set increments, so your exact total may round a hair from the estimate above.
Calculator no. 5
The pottery pricing calculator
What to charge for a piece, worked out from what it actually costs you. Materials, firing, your time at a rate you would accept from a boss, the overhead every piece has to carry, and a profit on top. Underpricing is the most common way potters lose money on work they sold.
Exactly $30.62 a piece. That is a fair wholesale price. A shop or gallery would retail it around $61.
Margin here is a true margin: at 20%, twenty cents of every dollar you charge is profit, which is not the same as adding 20% to your cost. Overhead is the share of studio fees, tools, glaze, packaging and time spent not making that each piece has to carry; 15 to 30% is common. Nobody knows your numbers but you, so change every one of these.
The firing calculator above gives you our rate for the firing line. Bisque is free here, so that line is only the glaze fire.
If you want the reasoning behind the shrinkage percentages these calculators use, how a pottery wheel actually works explains where the numbers come from, and is pottery hard to learn covers the timeline.
Calculator no. 6
The hours-to-friendship calculator
Friendship runs on hours. About 50 of them together makes casual friends, 90 makes friends, 200 makes close friends. Nobody schedules that. It happens when the hours pile up in the same room with the same people, and this room is very good at piling hours. Tell it how often you are in, and it tells you when the milestones land.
(4 classes × 3.5 hours) + (2 events × 3.5 hours ÷ 12 months).
When the milestones land (Hall’s three thresholds):
The math undersells it: hours are per person. Sit with a dozen regulars and one 3.5-hour class buys you three and a half hours with all twelve of them at once. You are not building one friendship here. You are building a crew.
That is 175 hours a year in the room. The hours only bank with the people actually in it, so pick a class night and keep it. Same night, same crew, same hours.
Milestones use Hall (2019), Journal of Social and Personal Relationships: about 50 hours together makes casual friends, 90 makes friends, 200 makes close friends. Past two years it rounds to the nearest half year, because promising you month 29 would be silly. The math assumes you actually show up.
Calculator no. 7
The shelf space calculator
Every level comes with one shelf, and every potter fills it. Here’s how many pieces yours actually holds before the pottery Tetris begins.
Your Green Level shelf: 24 × 12 × 8 inches (2,304 cubic inches).
2,304 cubic inches × 55% realistic packing ÷ 216 cubic inches per piece.
A 9″ piece doesn’t clear an 8″ shelf. Physics wins this one. Yellow Level and up clears it just fine, and your tall piece deserves the headroom.
The 55% is deliberate. Nobody packs a shelf like a shipping container, and mugs have handles. The full shelf survival strategy lives in our shelf space guide.
Calculator no. 8
The drying planner
Answer five things and we’ll tell you how to cover it, roughly how long it needs, and what’s most likely to go wrong. Winter stretches every number, the uncovered days included. Numbers are a starting point, not a promise. Your shelf is the real authority.
How to cover it: Bag over it, a little air left inside
Closed around the edges but not squeezed flat. That pocket of air is what takes it to leather hard and no further. This is your trimming setting.
Store it upside down once it is firm enough to lift, so the base gets air too. Your bats are plastic and the base gets nothing until you turn it over.
This is a guide, not a rule. Thickness beats every other input, and a genuinely thick piece can stay wet at its core long after the surface has gone pale. When the estimate and your gut disagree, believe your gut and give it another week. Nobody has ever regretted that.
The why behind every one of these answers lives in the full drying guide.
Calculator no. 9
The bag math calculator
A bag of clay is 25 pounds, or 27 if it is Berlin Gray. Here’s how far one stretches once you start pulling walls, and how many bags a whole set will take.
At 1.25 lb each, a 25 lb bag comes out even, with nothing left over.
12 pieces at 1.25 lb is 15 lb of clay, so one 25 lb bag with about 10 lb to spare.
Throwing weights are ballparks. Your mug is not our mug, and that’s kind of the point. The math also assumes every ounce ends up on the wheel, which is adorable of it. The clay weight calculator higher up will give you a real number for the piece you have in mind.
Calculator no. 10
The cone machine
Cones don’t measure temperature. They measure heatwork, which is temperature plus time. Same cone, three climb speeds, three different finish lines. Slide through Orton’s real chart.
We glaze fire to cone 5½: cone 5 with a 12-minute hold, which lands between the cone 5 and cone 6 rows. Bisque is cone 05. The same rate math above is why two firings never feel perfectly identical. Values are Orton’s published chart for self-supporting cones, measured over the final 180°F of the climb.
That’s the whole page
Enough math. Go make the thing.
This page will be right here holding all the numbers, every single time you need it. Bookmark it, use it for a decade. And if you’re not a member yet and somehow read this far: you clearly like knowing exactly what things cost. You are going to love it here.
The fine print, lovingly
Firing rates were accurate the day we built this page. If the page and the firing product ever disagree, the product wins.
Clay and kiln firings are in-studio, members-only purchases. Firing purchases from non-members are non-refundable, so please don’t buy one by accident.
Shrinkage numbers are manufacturer averages, and clay does what clay wants. Expect small batch-to-batch drift. Berlin Gray’s figure is our combination of the maker’s separate drying (5.7%) and firing (6.7% at 1300°C) numbers.
The clay weight, volume, drying and pricing calculators are estimates built on averages and on our own studio. Weigh, measure and price your own work; they are starting points.
The firing page charges in set increments, so exact totals can round slightly from the estimates here.