Measure the poster itself edge-to-edge — width, then height — and buy only a frame whose stated picture size matches those numbers, not the frame’s outside dimensions.
A print that reads 20 x 26 inches rarely measures exactly that, and the frame’s inner lip will quietly swallow another 2–3 mm of its edges. Skip the tape measure and you’ll order the wrong fit, pay return shipping, and start over. Here’s how to get it right the first time.
How Do You Measure A Poster Correctly?
Lay the poster flat on a clean, hard surface, smooth out any curls, and measure from edge to edge — width first, then height. Record both numbers as precisely as you can, down to the millimeter if your tape allows.
Adobe’s framing guidance follows the same logic: the frame size you choose should be based on the artwork’s exact dimensions, not on the exterior measurements of the frame. A print listed as 20 x 26 inches is a nominal size — a label, not a guarantee. Paper trims vary slightly between print runs, so the poster in your hands is the only measurement that matters.
Two habits keep this accurate:
- Use a rigid ruler or steel tape, not a soft fabric tape that droops across the middle.
- Measure at the center of each edge, not the corners, where a slight buckle can throw the reading off.
Write the numbers down or photograph the tape against the edge. You’ll want them when you compare frames, and you won’t want to re-measure a curled print twice.
What Does The Frame Size Actually Mean?
A frame labeled 20 x 26 holds a 20 x 26 picture — the nominal size refers to the artwork it fits, not the frame’s outer footprint. CustomFrames makes this distinction clear in its poster-framing guide: the advertised size is the opening the frame is built around.
This is the mistake that costs the most money. People measure an old frame on the wall, assume that’s their poster size, and order a duplicate. CustomFrames warns against exactly this — measure the poster itself, never an old frame, because the frame’s exterior dimensions include the molding thickness on every side.
One more wrinkle: actual measurements can vary slightly by frame style. Two frames both labeled 20 x 26 may have different inner lips, different molding widths, and slightly different opening tolerances. When a listing gives you a visible opening measurement, trust that number over the nominal label.
Mat Or No Mat — And The Lip Nobody Warns You About
Adding a mat means buying a larger frame, because the mat opening — not the frame’s outer size — is what has to match your poster. Mat openings are always smaller than the frame’s labeled size, since the frame has to hold both the mat and the print.
Frame Destination’s size guide illustrates the shift clearly: a matted 18 x 24 poster pairs with a considerably larger frame opening, because the mat adds border on all four sides. If you want a mat, decide that before you shop — it changes the frame size you need by several inches in each direction.
Even without a mat, the frame’s internal lip typically covers about 2–3 mm of the print’s edges. That’s normal and by design; the lip holds the poster against the backing. The practical consequence: keep critical text, signatures, or edition numbers away from the absolute edge, or they’ll disappear behind the frame.
| Setup | Frame Size To Buy | What Covers The Poster Edge |
|---|---|---|
| No mat, standard frame | Frame labeled 20 x 26 | Inner lip, about 2–3 mm per side |
| Poster measures 19.9 x 25.9 | Still a 20 x 26 frame | Lip absorbs the small gap |
| Poster measures 20.2 x 26.2 | Custom frame at true size | Standard 20 x 26 will not close |
| Single mat added | Larger frame; mat opening = 20 x 26 | Mat border plus lip |
| Wide decorative mat | Frame sized up several inches | Mat border widens further |
| Matted 18 x 24 poster | Larger opening, not the frame label | Mat opening governs the fit |
| Framing an old frame’s twin | Measure the poster instead | Old frame size misleads |
When To Buy Standard vs. Go Custom
If your poster measures within a millimeter or two of 20 x 26, a standard frame works and costs the least. Once you’ve confirmed the fit, a tested 20 x 26 poster frame comparison saves the guesswork of sorting through listings that use the same label for different builds.
If your print measures meaningfully over 20 x 26, or you’re adding a mat, go custom. Adobe’s framing guide notes that custom sizing is the route when standard dimensions don’t match, and 20 x 26 itself appears in custom-frame listings rather than on big-box shelves — which tells you it sits in the custom tier more often than the mass-market one.
Order in this sequence and you won’t reorder:
- Measure the poster flat, edge to edge, width then height. Write both numbers down.
- Decide on a mat before shopping, since it changes the frame size you need.
- Match the frame’s stated picture size or visible opening to your numbers — never the frame’s outside dimensions.
- Keep signatures and text clear of the outer 3 mm on each side.
- Confirm the listing states an opening measurement, not just a label, before you buy.
FAQs
Can I frame a poster slightly smaller than the frame size?
Yes, within reason. The frame’s inner lip overlaps roughly 2–3 mm of the print on each side, so a poster a millimeter or two under the nominal size still sits correctly and won’t shift. Anything more than a few millimeters undersized will look loose against the backing, and you’ll want a mat to center it properly.
Do I measure the poster or the old frame?
Always the poster. Measuring an old frame gives you the frame’s exterior dimensions, which include the molding on every side and run larger than the artwork. CustomFrames treats measuring the frame instead of the poster as a common and expensive mistake, since the resulting purchase won’t match the print you actually own.
Does adding a mat change what frame size I need?
Yes. A mat’s opening must match your poster, and the opening is always smaller than the frame’s labeled size because the frame holds both the mat and the print. Buying a mat means sizing up the frame accordingly, sometimes by several inches in each direction, before you order anything.
References & Sources
- Adobe. “How to Frame a Poster.” Confirms that frame size should be based on the artwork’s exact dimensions and that custom sizing covers non-standard prints.
- CustomFrames. “How to Frame Posters & Prints.” Source for measuring the poster flat, edge to edge, and for the warning against measuring an old frame.
- Frame Destination. “Guide to Frame and Mat Dimensions for Common Print Sizes.” Documents how matting shifts the required frame opening for a given print size.
