Check the space behind the frame lip
Use actual material measurements. The starting values are a worked example, not product specifications.
Worked example below. Calculate to check your own frame.
Worked example: a 10 mm frame recess with two mats.
Fits the entered depth, with 0.9 mm to spare
- Material stack
- 8.1 mm
- Depth needed, with allowance
- 9.1 mm
- Usable frame depth
- 10 mm
- Maximum backing thickness
- 3.9 mm
| Layer | Thickness |
|---|---|
| Glass or acrylic | 2 mm |
| First mat | 1.4 mm |
| Second mat | 1.4 mm |
| Photo / artwork | 0.3 mm |
| Backing and mounting boards | 3 mm |
| Closure allowance | 1 mm |
This is a thickness check. Confirm the actual frame, retaining hardware and material tolerances before assembly.
Maximum backing holds all other entries fixed; it is a space limit, not a board recommendation. The closure allowance is empty reserved space, not another material.
A thickness check, not an assembly guarantee. Inputs stay on this page and are not saved or included in tool analytics.
Explore custom portrait styles →What is picture frame rabbet depth?
The rabbet, also called a rebate, is the recess behind the frame’s front lip. Its depth is the front-to-back space available for the glazing, mats or spacer, artwork and backing. It is different from the frame’s outside height, the width of its decorative face, and the horizontal overlap that holds the picture in place.
Measure from the ledge where the front of the glass rests to the usable rear limit. Existing clips or tabs can reduce that space. Either measure to the usable closure position or enter the full recess depth and reserve space for the closure; do not subtract the same obstruction twice. Use your frame maker’s measurements when the profile is hard to measure.
Add the layers, then leave room to close the frame
Required depth = glazing + separation layer + artwork + backing + closure allowance. The separation layer is either the sum of one or two mats, a spacer’s effective air gap, or zero. The result subtracts required depth from usable rabbet depth to show spare space or a shortfall. Maximum backing thickness is the space left after every other entry, including the allowance.
For the worked example, 2 mm glazing + two 1.4 mm mats + a 0.3 mm photo + 3 mm backing makes an 8.1 mm material stack. Adding a 1 mm closure allowance requires 9.1 mm. A 10 mm recess leaves 0.9 mm to spare. These are editable example measurements, not standard thicknesses or a universal allowance.
Measure the real material or check its specification. Mat “ply” labels do not give an exact thickness. Include mounting boards and other backing layers once in the backing total. A mounted photo’s support belongs in either artwork thickness or backing, never both. If your construction combines mats with spacers, use a measured total separation thickness in spacer mode instead of counting an overlapping gap twice.
Use the result when choosing a frame for a portrait
A frame can be the right width and height yet too shallow for a double mat or thick backing. Compare the required depth against the maker’s usable rabbet depth before ordering. An exact mathematical fit leaves no additional tolerance; check the closure design and measurements. A deeper frame may be needed when the material stack exceeds the recess.
This calculation checks depth only. It does not certify secure assembly, conservation quality, glazing strength, or compatibility with a particular PortraitGift product. The American Frame glossary explains the framing components, and FrameTek’s spacer guide describes products that separate artwork from glass. Use the instructions for your actual frame and spacer.
For the opening around the picture, use the mat size calculator. For rail lengths and horizontal lip overlap, use the picture frame moulding calculator. Stretched canvas in an open floater frame uses a different construction: try the canvas floater frame calculator. Once your display is planned, explore turning a photo into a painting.
How it works
- Choose millimetres or decimal inches. Enter the usable depth behind the frame lip, then your measured glass or acrylic thickness.
- Choose mats, a spacer or no separator. Enter the photo, combined backing thickness and the space your closure needs. The second mat can be zero.
- Calculate to see the layer diagram, required depth and remaining space. Copy the plan and compare it with the frame maker’s specifications before buying.
Frequently asked questions
How do I calculate the rabbet depth I need?
Add the glazing, mat stack or spacer gap, artwork and backing thicknesses, then add the allowance required for your closure. For 2 mm glazing, two 1.4 mm mats, a 0.3 mm photo, 3 mm backing and a 1 mm allowance, the required depth is 9.1 mm.
Is rabbet depth the same as rabbet width?
No. Depth measures front-to-back space for the layers. Width measures the horizontal overlap of the frame lip. This calculator checks depth; the moulding calculator uses the overlap width to calculate rail lengths.
Can I check a double mat or a spacer?
Yes. Mat mode adds the first and second mat thicknesses; set the second to zero for one mat. Spacer mode uses an effective gap between glazing and artwork instead. For a combined construction, enter its measured total separation in spacer mode and include each physical layer only once.
What if the backing fits but there is no room for the clips?
The closure allowance reserves space beyond the material stack. The calculator distinguishes a stack that protrudes from one that fits but leaves too little closure space. Check the retaining hardware or choose a deeper frame; the calculator does not recommend compressing the artwork or glazing.
Are the example thicknesses standard sizes?
No. They are a worked example. Measure your materials or use the supplier’s specifications, including the actual mat thickness rather than a ply label. The closure allowance depends on your frame and hardware.
Does a fit result mean my photo is framed correctly?
It means the entered thicknesses fit the entered depth budget. Width, height, glazing strength, artwork separation, material suitability and secure assembly require separate checks. With glazing and no separator, the tool also reminds you to check whether direct contact is suitable.
Is this free, and are my measurements saved?
The calculator, diagram and copyable 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 →