Online RulerHow to measure envelope flap width
Measure the flap from left to right where it joins the back of the envelope. Open it and use the two points where the fold meets the flap edges. That span gives you the width at the fold. If you need to know how far the flap reaches down the back, measure from the fold to the tip as a separate dimension.
- The fold gives you a straight reference even when the flap has sloped or rounded sides.
- For a label or seal, measure the available width at the height where you plan to place it.
- A paper strip can transfer the span to a calibrated AR Ruler screen when the envelope is wider than the scale.
- Set the envelope back side up on a table. Open the flap so you can see both ends of its fold. Let the paper rest flat without pulling on the corners.
- Lay a ruler along the fold. Line up the first endpoint with zero and read the mark at the other endpoint. Look straight down so you can see where each edge meets the scale.
- If the fold is faint, lay a narrow strip of scrap paper along it. Mark both endpoints on the same edge of the strip. Keep the pencil marks fine so you have clear points to read.
- To read the strip on a calibrated AR Ruler screen, line up one pencil mark with zero at the left end. Move the marker to the second pencil mark and read the span. Record the value with its unit and note that you measured at the fold.
Find the fold ends
Look for the crease that bends when you close the envelope. Follow it outward until it meets each side of the flap. On a rounded corner, use the point where the straight crease meets the curved edge. A side seam may sit close to that point. Trace the crease with your eyes to see where it actually ends.
Check room for a seal
Close the flap and place the seal where you want it to sit. If the flap tapers, check the space across the seal near its top and bottom edges. Use the smaller span when deciding what will fit. If the seal should cross onto the envelope body, check that it can lie flat on both surfaces.
Use a printed scale
The AR Ruler printable page has a 15 cm scale and a 6 in scale. Print at actual size and check the 10 cm test line with a physical ruler before using the sheet. Place the printed scale beside the envelope so the paper can stay flat as you read it.
Where do I start on a pointed flap?
Start at either end of the fold for width. Use the point of the flap when measuring its depth.
Can I measure the envelope body instead?
Measure the flap itself. Its edges can meet the fold inside the outer edges of the envelope body.
What if my screen scale is too short?
Transfer the endpoints to a paper strip and read them against a longer physical ruler or a printed scale that spans both marks.