How to Get True Scale From a Photo Scan

On most object scans, true scale is a two-step job: capture the photos, then measure something on the real object and type it into the Rescale tool. An AprilTag takes over the second step. The check before you cut or order anything still belongs to a tape or calipers.

Getting true scale from a photo scan comes down to giving the model one known measurement. A printed Polycam AprilTag is that measurement. Polycam knows the tag's exact printed size, finds it in your photos, and scales the whole capture from it, with accuracy typically greater than 99%.

Automatic scaling works on photogrammetry captures for Business and Enterprise accounts. Anyone can download and print the tags, but on other plans, the tag is just photographed like any other object and the model won't be scaled. This feature is available on iOS, Android, and the web.

Key Takeaways

• Only have one printed AprilTag in at least eight photos to scale a photogrammetry capture automatically.
• Print at 100%, check the printed scale line with a ruler, and mount the tag on a flat, sturdy surface.
• Use the 4 × 6 in (A6) tag for small objects, and 8.5 × 11 in (A4) or larger for window-sized scenes and up.
• Check critical dimensions with a tape or calipers before you order or cut anything.

Why a Photo Scan Has No True Scale on Its Own

A LiDAR scan measures distance as you move, so the model comes out at real size. A photo scan instead finds the same points across many photos and works out the shape from how those points shift between shots. This process will get the proportions right, but nothing in the photos says whether the object is two inches across or two feet.

The fix is to give the model one real measurement. An AprilTag puts that measurement in the photos, so the scaling happens while the capture processes.

What an AprilTag Is

An AprilTag is a high-contrast square marker that looks a bit like a QR code. Software can find its four corners precisely. Because Polycam knows the exact printed size of each tag, it compares that size with how the tag appears in your photos and works out the true scale of the whole capture.

Which Tag Size to Use

Polycam has to see the tag's corners clearly to measure it, so the bigger the scene, the bigger the tag. The 4 × 6 in (A6) tag works for small objects and close-range captures. For anything window-sized or larger, use a tag with a printed scale of 6 inches or more, which means 8.5 × 11 in (A4) and up. If you're not sure, size up, because a tag larger than needed still scales accurately, and one that's too small won't resolve.

What You'll Need

• A Polycam Business or Enterprise account for automatic scaling
• A printer that can print at 100% scale, and paper to match the tag size
• A ruler to check the printed scale line
• Cardstock, foam board, or another rigid backing
• A tape measure to spot-check

How to Get True Scale From a Photo Scan, Step by Step

• Download the tag that fits your scene. Pick the size from the table above. Imperial and metric versions are both in the help center.
• Print the tag at 100%. Set your printer's scaling to 100% and turn off options like "Fit to page" or "Shrink to fit." These settings change the tag's size, so the model will be off by the same amount.
• Check the printed scale line. Each tag has a line underneath labeled with its length, like "6 inches." Use a ruler to check it, and reprint if it doesn't match. Printer settings can reset to "Fit to page" between jobs, so check every new batch.
• Mount it flat. Fix the tag to cardstock or foam board. A curl or a wrinkle changes the tag's shape in the photos and throws off the measurement.
• Put one tag in the scene. Place it where you'll see it from several angles as you move around the subject. Make sure no other tag is visible anywhere.
• Take photos as you normally do. The tag should appear in at least eight photos from different angles, with all four corners in the frame and in focus. These photos count toward your usual coverage, so you don't need to photograph the tag separately.
• Check the result. After processing, measure something in the scene that you already know. If the tag was printed at 100%, kept flat, and appears in eight or more photos, accuracy is usually better than 99%. For example, on a 24-inch cabinet door, that's within about a quarter inch.

If Your Scan Didn't Scale

The most common reason a capture isn't scaled is a second tag somewhere in the scene. Check for that first. Then, see how many photos the tag appears in and whether it was large enough for the scene. If the tag's corners are cut off or out of focus, scaling won't work.

If the model is scaled but the measurements are off, the issue is usually with the print or how the tag was mounted. Always double-check the scale line on the tag again. If it doesn't match the label, the tag printed at the wrong size. If it does match, check if the tag bent or curled during your scanning session.

Using Rescale When You Don't Have a Tag

You can still use the Rescale tool for scans without a tag or on plans without automatic scaling. Measure a distance on the real object, as long as possible, between two points you'll recognize in the model. Then open the capture in the app or on the web, select Rescale, set the same two points on the model, and enter the distance.

Using a longer reference is worth the effort. An eighth-inch error on a 6-foot reference barely affects the model, but the same error on a 6-inch reference can throw off every dimension by about 2%.

When to Still Reach for the Tape

A scaled model is accurate enough for planning, quoting, and checking if something fits. But if you're ordering parts, cutting materials, or sending dimensions to a fabricator, always measure the critical dimensions on the real object first. Even with over 99% accuracy, the error increases with the size of what you scanned. A quarter inch on a cabinet door might not matter for layout, but it could mean a replacement door won't close.