Kiln Load Calculator
How many pieces fit on the kiln shelves, how many firings a batch needs, and the firing cost per piece.
A firing costs the same whether the kiln is full or half empty, so the cost per piece is decided by how well the load is planned. This calculator works out how many pieces fit on the shelves you have, how badly the packing fraction and the gaps cost you, how many firings the batch needs and what each piece carries of the firing cost. It is planning arithmetic for a load, not a loading instruction: the kiln manual and the shelf manufacturer decide what is safe to stack.
At a glance
- What you enter
- Shelf length, Shelf width, Shelves you can load in one firing, Shape of the pieces, Your own packing fraction, Footprint of one piece, Height of one piece, Gap between pieces, Pieces in the batch, Cost of a bisque firing, Cost of a glaze firing, Minutes to load and unload one firing, Your time per hour
- What you get
- Area of one shelf, Total shelf area in one firing, Usable shelf area, Pieces per firing, Pieces per shelf, Firings for the batch, Pieces in the final firing, How full the final firing is, Cost of both firings for one load, Firing cost for the whole batch, Firing cost per piece in a full load, Firing cost per piece across the batch, Firing and loading cost per piece, Labour cost for the whole batch, Share of the shelf that stays empty, The final firing is under sixty percent full
- Formula shown
- Yes, every step, right below your results.
- Cost and privacy
- Free, no account needed, and nothing you type is stored.
See worked examples ↓ Read the FAQ ↓ Share this calculator ↓
Results are estimates for general information only, always verify with a professional or official source before acting on them.
How to use the Kiln Load Calculator
- 1 Enter Shelf length (cm)
- 2 Enter Shelf width (cm)
- 3 Enter Shelves you can load in one firing, Count the shelf positions the kiln actually has, including any you can stack on props.
- 4 Enter Shape of the pieces
- 5 Enter Your own packing fraction (%), Leave at zero to use the figure for the shape above. Measure a real load once and use that figure.
- 6 Enter Footprint of one piece (cm), The widest dimension of the footprint, which is usually the rim of a bowl or the width of a mug handle.
- 7 Enter Height of one piece (cm), Used to check the load against the usable height of the kiln.
- 8 Enter Gap between pieces (cm), Two centimetres is a working figure for small ware, more for a wide foot or a runny glaze.
- 9 Enter Pieces in the batch
- 10 Enter Cost of a bisque firing, Electricity or gas, plus a share of element wear for the firing.
- 11 Enter Cost of a glaze firing
- 12 Enter Minutes to load and unload one firing (min), Used for the labour figure. Loading carefully takes longer than loading fast and breaking things.
- 13 Enter Your time per hour, Optional. Used for the labour cost of loading and unloading.
- 14 Press Calculate. You get Area of one shelf, Total shelf area in one firing, Usable shelf area, Pieces per firing, Pieces per shelf, Firings for the batch, Pieces in the final firing, How full the final firing is, Cost of both firings for one load, Firing cost for the whole batch, Firing cost per piece in a full load, Firing cost per piece across the batch, Firing and loading cost per piece, Labour cost for the whole batch, Share of the shelf that stays empty, The final firing is under sixty percent full, and every intermediate step stays visible under "How is this calculated?"
- 15 Change one number at a time to see what moves the result, that is usually the most useful thing a calculator can tell you.
Worked examples
Real numbers run through this calculator. Open any example to load it into the form above and change it.
Three 40 cm shelves of mugs
Twelve centimetre round pots with a 2 cm gap, a 120 piece batch.
You enter
- Shelf length
- 40 cm
- Shelf width
- 40 cm
- Shelves you can load in one firing
- 3
- Shape of the pieces
- 1
- Your own packing fraction
- 0 %
- Footprint of one piece
- 12 cm
- Height of one piece
- 10 cm
- Gap between pieces
- 2 cm
- Pieces in the batch
- 120
- Cost of a bisque firing
- $8.00
- Cost of a glaze firing
- $12.00
- Minutes to load and unload one firing
- 45 min
- Your time per hour
- $15.00
You get
- Area of one shelf
- 0.16 sq m
- Total shelf area in one firing
- 4,800.00 sq cm
- Usable shelf area
- 3,744.00 sq cm
- Pieces per firing
- 19
- Pieces per shelf
- 6
- Firings for the batch
- 7
- Pieces in the final firing
- 6
- How full the final firing is
- 31.6%
- Cost of both firings for one load
- $20.00
- Firing cost for the whole batch
- $140.00
- Firing cost per piece in a full load
- $1.05
- Firing cost per piece across the batch
- $1.17
- Firing and loading cost per piece
- $1.82
- Labour cost for the whole batch
- $78.75
- Share of the shelf that stays empty
- 22.0%
- The final firing is under sixty percent full
- 1
Nineteen pieces per firing, so seven firings for 120 mugs, and 2.86 per mug for the firing plus 0.94 for loading.
Firing a batch of flat tiles
Fifteen centimetre tiles with a 2 cm gap on the same shelves.
You enter
- Shelf length
- 40 cm
- Shelf width
- 40 cm
- Shelves you can load in one firing
- 3
- Shape of the pieces
- 3
- Your own packing fraction
- 0 %
- Footprint of one piece
- 15 cm
- Height of one piece
- 2 cm
- Gap between pieces
- 2 cm
- Pieces in the batch
- 100
- Cost of a bisque firing
- $8.00
- Cost of a glaze firing
- $12.00
- Minutes to load and unload one firing
- 30 min
- Your time per hour
- $15.00
You get
- Area of one shelf
- 0.16 sq m
- Total shelf area in one firing
- 4,800.00 sq cm
- Usable shelf area
- 2,640.00 sq cm
- Pieces per firing
- 9
- Pieces per shelf
- 3
- Firings for the batch
- 12
- Pieces in the final firing
- 1
- How full the final firing is
- 11.1%
- Cost of both firings for one load
- $20.00
- Firing cost for the whole batch
- $240.00
- Firing cost per piece in a full load
- $2.22
- Firing cost per piece across the batch
- $2.40
- Firing and loading cost per piece
- $3.30
- Labour cost for the whole batch
- $90.00
- Share of the shelf that stays empty
- 45.0%
- The final firing is under sixty percent full
- 1
Flat tiles pack at 55 percent, so thirteen per firing and eight firings for 100 tiles. Flat work is the worst case for kiln cost per piece.
A small batch with a nearly empty last load
Twenty four bowls on three shelves, a batch of 45.
You enter
- Shelf length
- 40 cm
- Shelf width
- 40 cm
- Shelves you can load in one firing
- 3
- Shape of the pieces
- 1
- Your own packing fraction
- 0 %
- Footprint of one piece
- 12 cm
- Height of one piece
- 10 cm
- Gap between pieces
- 2 cm
- Pieces in the batch
- 45
- Cost of a bisque firing
- $8.00
- Cost of a glaze firing
- $12.00
- Minutes to load and unload one firing
- 45 min
- Your time per hour
- $15.00
You get
- Area of one shelf
- 0.16 sq m
- Total shelf area in one firing
- 4,800.00 sq cm
- Usable shelf area
- 3,744.00 sq cm
- Pieces per firing
- 19
- Pieces per shelf
- 6
- Firings for the batch
- 3
- Pieces in the final firing
- 7
- How full the final firing is
- 36.8%
- Cost of both firings for one load
- $20.00
- Firing cost for the whole batch
- $60.00
- Firing cost per piece in a full load
- $1.05
- Firing cost per piece across the batch
- $1.33
- Firing and loading cost per piece
- $2.08
- Labour cost for the whole batch
- $33.75
- Share of the shelf that stays empty
- 22.0%
- The final firing is under sixty percent full
- 1
Two full firings of nineteen and a final one of seven, which is 37 percent full and flagged as worth merging with the next batch.
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Frequently asked questions
How many pieces fit in a kiln load?
Take the total shelf area, apply a packing fraction of about 70 to 80 percent for round domestic ware, then divide by the footprint of one piece plus its gap. Three 40 by 40 centimetre shelves is 4800 square centimetres, which at 78 percent is 3744 usable, and a 12 centimetre pot with a 2 centimetre gap occupies 196 square centimetres as a square, so nineteen pieces per firing. The corners of a square shelf and the shape of the pieces are why it is not the full 33.
Why is my firing cost per piece so high?
Because a firing costs the same whether it is full or nearly empty. If a bisque and a glaze firing together cost 20, a full load of 80 mugs carries 25 pence each, while a two thirds full load of 53 carries 38 pence each. The biggest lever is batching: hold smaller runs back and fire them together. The second lever is packing, which is worth checking by measuring a real load once rather than assuming.
What is a realistic packing fraction for a kiln shelf?
About 78 percent for round pots and bowls in a tight but workable arrangement, 65 percent for oval or rectangular dishes, 55 percent for flat tiles that cannot interlock, and around 70 percent for a typical mixed domestic load. Round pieces on a square shelf always leave the corners empty, and every piece needs a gap so it can be lifted clear and so glaze cannot fuse two pieces together.
Should I fire a half full kiln?
Usually not, unless you have a deadline. The fixed cost of a firing is spread over the pieces in it, so a half full load costs nearly twice as much per piece as a full one. This calculator flags when the final firing of a batch would be under sixty percent full, which is the point at which holding the ware back for the next batch is usually the cheaper decision, provided you have somewhere safe to store it green or bisqued.
How do I work out what a firing costs?
Add the electricity or gas for the firing to a share of the element or burner wear, then add your own time for loading and unloading. Then divide by the pieces in the load. If your kiln has a meter, one or two measured firings will give you a far better figure than any estimate, because consumption depends on the kiln, the schedule, the insulation and how much thermal mass is in the load. Loading practice and maximum temperatures must follow the kiln manufacturer instructions.