How to Measure a Floor Plan From a Photo

A photo of a printed floor plan is a perfectly good source for room sizes and square footage, as long as you undo two things the camera adds: perspective, which shrinks whatever is farther away, and an unknown scale. Measuring an actual room from a photo of the room is a harder problem; it's covered near the end.

1. 1

   Take a good photo

   Flat paper, even light, phone held parallel to the sheet, drawing filling the frame. Details below.
2. 2

   Straighten the perspective

   Mark the four corners of something you know is rectangular. The photo is remapped so it's square again.
3. 3

   Set the scale

   Click both ends of a long printed dimension and type its length. No dimensions? Use a door: a 3'0" door is 36 in wide.
4. 4

   Check, then measure

   Measure a second known dimension running the other way. If it agrees within 1–2%, measure away.

How to photograph a floor plan

• Flat. Lay the sheet on a table and weigh down the corners. Curls and folds bend lines in ways perspective correction can't undo.
• Even light. Daylight or two lamps from the sides. Avoid your own shadow and flash glare on glossy paper.
• Square on. Hold the phone parallel to the sheet, centered over it. The straighter the photo, the less correction it needs.
• Fill the frame with the drawing, but keep the four corners of the sheet (or of the building outline) and a printed dimension in each direction in the shot.
• Use the main camera at full resolution rather than the ultra-wide lens, which typically bends straight lines near the edges. Take two or three shots and keep the sharpest.

A flatbed scanner, or a phone's document-scan mode, is better still. Best of all is the original PDF: ask the builder, architect or listing agent for it, since a PDF keeps exact lines and printed text.

Why does perspective correction matter?

In a photo taken at an angle, the far side of the plan comes out smaller than the near side, so no single scale fits the whole picture. If the far edge of the sheet looks 10% shorter than the near edge, a scale set on a dimension near you will read walls on the far side about 10% short. Rectangles turn into trapezoids, and areas go wrong with them.

Perspective correction fixes this. You mark the four corners of something you know is rectangular, and the whole photo is remapped so that rectangle is square again, along with everything else on the flat sheet. Pick the largest rectangle you can: the sheet border, the title block frame, or the building outline if it's a plain rectangle. If you know its real size (the outline is 48' × 36'), entering it sets the scale at the same time.

Setting the scale from a dimension or known object

Scale is real length divided by length in the image, exactly as on a PDF drawing. What you choose as the reference decides most of the accuracy, because your clicking error stays about the same size whatever you click: missing by 2 in on a 36-in door is a 5.6% error, but on a 30-ft overall dimension it's 0.6%.

Door sizes are often written on the plan, either as 3'0" or as a code like 3068 (3'-0" wide, 6'-8" tall). Use the written size rather than assuming one. See how to read floor plan dimensions for the notation. If the photo came from a stretched scan or fax, set a separate horizontal and vertical scale.

How accurate is measuring from a photo?

We measured the same sample plan from the original PDF and from a phone photo taken at an angle:

On a 200 sq ft bedroom, 1–2% is 2–4 sq ft; on a 1,800 sq ft house, 18–36 sq ft. That's plenty for checking a listing's size, planning a remodel or ordering flooring with a waste allowance. For an appraisal or a dispute, get the original PDF or measure on site. Keep in mind that the photo can only be as right as the plan itself, and plans don't always match the house as built.

Measuring a room from a photo with a reference object

You can also measure a real room or wall from a photo, with limits. It only works for things lying in one flat surface, with a reference of known size in that same surface. A door photographed straight on works for the wall it's in: if the 36-in door is 180 pixels wide, each pixel is 0.2 in, and a wall 1,020 pixels long is 204 in, or 17 ft. A block of floor tiles you've measured works for the floor, and with four corners it can also correct the perspective.

What throws a room photo off:

• Anything not in the same surface as the reference, such as furniture or a wall at a different depth.
• Furniture hiding the corners and the floor edges you need.
• Wide-angle lenses, which bend straight lines, especially near the edges.
• Whole rooms rarely fit in one shot at a useful angle.

For room dimensions you'll order materials from, a tape or laser measure is faster and more accurate. Sketch what you measure and add up the area with the square footage calculator.

Limits worth knowing

• FloorFigures doesn't read text in photos, so on a photo you set the scale and name the rooms yourself. Automatic scale and room names work on PDFs that contain text.
• Low-resolution photos can make small dimension text unreadable. Shoot the plan in sections if you have to.
• Perspective correction needs a true rectangle. A building outline with jogs won't do; use the sheet edge.
• A photo of a plan on a screen adds glare and pixel patterns. Ask for the file instead.

Measuring a photo in FloorFigures

FloorFigures is free and runs in your browser, on a phone or a computer. Your photo stays on your device; nothing is uploaded.

1. Open the photo: JPG, PNG, WebP or HEIC, straight from your phone's camera roll.
2. Straighten it with the perspective tool (P): click the four corners of a rectangle, clockwise from top left, and optionally enter its real size to set the scale in the same step.
3. Set the scale (S) by clicking both ends of a printed dimension and typing its length, or pick a standard object such as a door, tub or counter.
4. Check it (K) on a second dimension, then click inside a room (M) to trace it, or measure any distance (D).