Size the glazing for your frame opening
Measure the actual frame. The starting values are a worked example, not product specifications.
Worked example below. Calculate to size your own frame.
Worked example: a 306 × 406 mm frame recess with 2 mm glass.
Cut to 303 mm × 403 mm — nearest standard sheet is 12 × 16 in
- Cut size, all sheets
- 303 mm × 403 mm
- Fitting gap each side
- 1.5 mm
- Lip coverage (centred)
- 5.5 mm
- Approx. sheet weight
- ≈0.61 kg (1.35 lb)
| Piece | Cut size |
|---|---|
| Glazing (glass) | 303 mm × 403 mm |
| Mat board, outer size | 303 mm × 403 mm |
| Backing board | 303 mm × 403 mm |
A standard 12 × 16 in sheet is large enough for this cut but needs 1.8 mm removed from its width and 3.4 mm from its height. Ask your supplier whether these trims are practical for the material and thickness.
The mat window is a separate decision — this is the outer size only. Weight covers the glass sheet alone at a nominal density, not the finished frame.
Gap and lip coverage assume a centred sheet in a square frame with equal lips. Check the narrowest recess measurements and allow for movement before ordering.
A sizing check, not a glazing or safety guarantee. Inputs stay on this page and are not saved or included in tool analytics.
Explore custom portrait styles →How picture frame glass size is measured
Start with the measured recess, not the frame’s label, sight opening or outside dimensions. It is cut to fit the rabbet — the recess behind the frame’s front lip. Measure the rabbet opening from side to side and top to bottom at the back of the frame, where the sheet will actually sit. The sight opening you see from the front is smaller, because the lip overlaps the recess on every side.
For a simple flat stack, this planner uses one shared outer size for the glass or acrylic, each mat board, and the backing. This calculator returns that shared cut size, the gap it leaves around the sheet, how much of the sheet the lip still covers, and the nearest common pre-cut sheet size. The calculation assumes a square frame with equal lips and a centred sheet. Frames and component requirements vary; measure the narrowest recess width and height and confirm the supplier’s specifications.
The fitting allowance, and why the sheet is cut small
A sheet cut to the exact opening can jam, refuse to seat, or crack as the frame and glazing move with temperature and humidity. Framers therefore cut the sheet slightly smaller than the opening — the total fitting allowance, split evenly on each side. Acrylic moves more than glass, so acrylic sheets generally want a more generous allowance; check the sheet maker’s guidance.
For the worked example, a 306 × 406 mm rabbet opening with a 3 mm total allowance gives a 303 × 403 mm cut, a 1.5 mm gap on each side, and — with a 7 mm lip — 5.5 mm of lip coverage holding each edge. These are editable example measurements, not standards. The allowance can also be too big: if the gap on one side reaches the lip overlap, the lip no longer holds the sheet, and the calculator warns instead of pretending the fit works.
From cut size to a finished framed portrait
The cut size answers “what do I order?” — the other framing decisions have their own calculators. The window in the mat is picture-plane geometry: use the mat size calculator. Whether the whole stack fits front-to-back is a depth budget: use the rabbet depth calculator. Rail lengths for building the frame come from the moulding calculator, and the approximate sheet weight from this page contributes to the total frame weight needed by the hanging hardware calculator.
This calculation is geometry only. It does not certify glazing strength, UV protection, conservation practice, or that any listed sheet size is in stock at your supplier. The American Frame glossary explains rabbet, lip, glazing and the other framing terms used here.
If the frame is destined for a portrait you have not made yet, the fun part comes first: turn a photo into a painting, or browse finished custom portrait styles below.
How it works
- Choose millimetres or decimal inches, then enter the rabbet opening’s width and height measured at the back of the frame, and the lip overlap.
- Choose glass or acrylic, enter the sheet thickness, and set the total fitting allowance the sheet should leave in the opening.
- Calculate to get the shared cut size for glazing, mats and backing, the gap and lip coverage per side, the approximate sheet weight and the nearest standard sheet. Copy the plan for your glazier or supplier.
Frequently asked questions
What size glass do I need for my picture frame?
Measure the rabbet opening — the recess behind the front lip, at the back of the frame — and subtract a total fitting allowance. In the worked example, a 306 × 406 mm opening with a 3 mm allowance takes a 303 × 403 mm sheet. Mats and backing are cut to the same outer size.
Why is picture frame glass cut smaller than the opening?
The frame and sheet expand and contract at different rates, and a sheet with no gap can jam or crack. The allowance splits into a small gap on each side. The lip still has to hold the sheet, so the calculator also reports the remaining lip coverage and warns when the gap swallows it.
Is the frame’s nominal size the same as the glass size?
The nominal size can match a standard glass sheet, but it is not the measured recess size. An “8 × 10” frame generally accepts an 8 × 10 in stack; the rabbet opening may be larger and the sight opening smaller. Measure the actual recess. The standard-sheet suggestion compares your cut size against common pre-cut sizes such as 8 × 10 in or A4.
Do the mats and backing use the same cut size as the glass?
For a simple flat stack, this calculator uses one shared outer size. Check your frame and supplier’s requirements before using that size for every component. Only the mat’s window differs; plan the window and borders with the mat size calculator.
Does the calculator treat glass and acrylic differently?
The geometry is identical. Acrylic expands more with heat and humidity, so it typically deserves a larger allowance — the tool reminds you to check the sheet maker’s figure rather than inventing one. The weight estimate uses each material’s nominal density and is approximate.
Can I use a standard-size sheet instead of a custom cut?
Often. The result names the smallest common sheet that covers your cut size — sometimes turned sideways — and how much must be trimmed off. Ask your supplier to confirm the cut size, material tolerance and whether the required trim is practical. When nothing on the list is big enough, order a custom cut.
Is this free, and are my measurements saved?
The calculator, diagram and copyable cut plan are free with no signup or upload. Measurements stay in page memory and are not saved, added to URLs or included in tool analytics. Shared site analytics record basic usage actions and a visitor identifier.
Explore custom portrait styles →