The Formula, Stated Simply
For each side of a picture frame, the cut length is: the artwork dimension, plus twice the moulding width, plus a small allowance. Multiply by four — two width cuts, two height cuts — and you get the total moulding needed for the frame. If you're cutting from the same stick, add kerf between each cut for the material the saw blade removes.
Cut Length Formula
cut length = artwork dimension + (2 × moulding width) + allowance
Each term has a specific reason for being there. Let's break them down.
The Artwork Dimension
This is the inside opening of the frame — the size of the artwork the frame will hold (or the mat opening, if the frame is matted). When a customer says "I have a 16×20 print," the artwork dimension is 16×20. This is the rabbet-to-rabbet measurement — the opening where the glass and artwork sit.
One common confusion: if the artwork is matted, the artwork dimension is the outer mat size, not the original artwork size. A 16×20 print with a 3" mat has a mat outer of roughly 22×26, which is the "artwork dimension" that feeds into this formula. For the mat math, see the mat border calculator.
The Moulding Width
The face width of the moulding profile, measured from the inside edge of the rabbet to the outer edge. A narrow "slip" moulding might be 0.5"; a standard custom moulding is usually 1"–2"; a chunky gallery moulding can exceed 3".
You add 2×moulding width to each cut because the moulding extends across both sides of the miter. Picture it: at each corner, the 45-degree miter creates a joint where the moulding on one rail ends exactly where the adjoining rail begins. But the moulding has thickness (the width), and that thickness is present on both ends of each piece. So the cut has to include the artwork dimension plus one moulding width on the left and one on the right.
The Allowance
A small amount of extra length — typically 0.125" (1/8") — added to each cut so the frame opening is slightly larger than the artwork. Without it, the frame would be an exact fit, which means the glass package (glass + mat + backing) would bind as temperature and humidity change. The allowance is your safety margin.
Different shops use slightly different allowances. 1/8" per cut is the most common. Some shops use 1/16" on small or precision frames. The exact number matters less than consistency — pick one and stick to it. See the glossary for a longer definition.
A Worked Example
Say you're framing a 16×20 artwork with a 1.5" moulding and the standard 0.125" allowance. Here's the cut math, step by step:
Frame: 16×20 artwork, 1.5" moulding, 1/8" allowance
That's the frame moulding itself. Now factor in kerf.
Adding Kerf
Kerf is the width of material the saw blade removes with each cut. For a standard picture framing miter saw blade, it's 0.125" (1/8"). If all four cuts come from the same 120" stick, you make four cuts. But — and this is the rule that trips people up — you only consume kerf between cuts, not before the first cut.
Four cuts on the same stick = three "between cuts" kerfs = 3 × 0.125" = 0.375" of kerf.
Adding kerf
From a standard 120" stick, you'd have 120 − 84.875 = 35.125" of leftover moulding. That's a remnant — usable for a smaller frame if you track it. Most shops don't, which is why remnants pile up untracked and eventually become waste. Read more about that in "How to Reduce Moulding Waste".
The Quick Approximation Formula
If you need a rough estimate without a calculator, there's a shortcut:
Quick approximation
total moulding ≈ 2 × (W + H) + (8 × moulding width)
The first term is the perimeter (4 cuts of the artwork dimensions, minus the overlap at the corners — which nets out to 2×perimeter). The second term is the miter allowance at all four corners. This gives you a ballpark number for estimating or quoting — but not precise enough for actual cuts.
For the 16×20 example: 2×(16+20) + 8×1.5 = 72 + 12 = 84". The exact answer (with allowance and kerf) is 84.875" — so the quick formula undershoots by 7/8". Fine for a rough quote, but always use the full formula before cutting.
What the Formula Doesn't Tell You
This formula is correct for one frame from one stick. It's the right math for the simplest case. But most real shops don't cut one frame at a time from a dedicated stick. They have a batch of work orders, multiple profiles, a rack of remnants from yesterday, and varying stock lengths in inventory.
Once you have multiple orders sharing a profile, the formula is only the starting point. The harder question is: how do you arrange all these cuts across your available sticks so you waste the least material? That's an arrangement problem, not a cut-length formula.
Picture a day with 15 orders across 3 profiles and 5 different stock lengths on the shelf, plus a dozen remnants. The number of reasonable cut arrangements grows fast — faster than anyone can sort through by hand. A human with a spreadsheet can evaluate maybe 10 of them. A purpose-built optimizer returns a strong plan in a fraction of a second.
Read more on this in "Cut Optimization Explained" or see how RailChop's optimizer handles it.
Edge Cases and Variations
Rabbet vs. Point-to-Point Measurement
The formula above assumes rabbet-to-rabbet measurement — the inside opening where the glass sits. If your shop measures point-to-point (the outside dimension of the finished frame), the formula changes: the moulding-width term disappears because it's already baked into the point-to-point number. Full breakdown in "Rabbet vs. Point-to-Point".
Frames with Multiple Moulding Layers (Stacking)
Some frames stack two or more mouldings — an inner fillet around the artwork, with an outer frame moulding. Each layer uses the same formula, but the inner layer's "artwork dimension" is the rabbet of the outer frame (or vice versa, depending on the design). Compute each layer separately and track the total moulding for each profile.
Non-Rectangular Frames
The formula is for four-sided mitered frames. Octagonal, oval, and hexagonal frames use different math — each segment has its own angle calculation and a different miter allowance per joint. Most framing software (including RailChop) handles rectangular frames only; specialty shapes are a separate workflow.
Canvas Stretcher Bars vs. Picture Frame Moulding
Stretcher bars use a different formula because they don't have mitered corners — they have interlocking joints. The formula above doesn't apply. Stretcher bar calculations are straightforward (length = canvas dimension + bar width), but they're a different product category.
A Note on Fractions
Everything above uses decimal inches for clarity. In a shop, you'll usually see fractions: 19-1/8", 23-1/8", 1/8" for kerf and allowance. The math is identical — 0.125 is the same as 1/8 — but fractions are easier to read on a cut ticket. RailChop displays results in both formats. If you prefer one over the other, set it once in settings and the cut plan uses that throughout.
Putting It Together
For a single frame from a fresh stick, the formula is complete as stated. For a day of orders across multiple profiles and remnants, the formula is only the starting point — you need an optimizer to arrange cuts efficiently across sticks. Use the moulding calculator for single-frame math. Use a cut optimizer for the day's full batch.
The takeaway: cut length = artwork dimension + 2×moulding width + allowance. Add kerf between cuts on the same stick. Multiply by four for a standard frame. For one frame, that's all you need. For a day's orders, you need a multi-order optimizer — the formula alone can't tell you how to pack cuts across sticks efficiently.
On the App Store today.
RailChop is a cut optimizer built for picture framers. Free to start — the 14-day trial unlocks every Business feature.
Android user? Get notified
We’ll email you when the Android app is live. No spam.