Height from a Photo Calculator
Mark the horizon, something whose height you know, and the thing you want measured. The perspective does the arithmetic.
How the geometry works
Every vertical thing standing on the ground has one point on it that is exactly as high as the camera: the point where the horizon crosses it. A 2 m door photographed by someone 1.6 m tall has the horizon crossing it at 1.6 m, four-fifths of the way up, wherever the door is in the picture, near or far. That is the whole trick. Mark the base and top of the door and the horizon, and the fraction of the door below the horizon gives the camera height. Mark the base and top of a person and the same fraction, now with the camera height known, gives theirs.
Because the rule holds at any distance, the reference and the unknown do not need to be side by side. What they need is the same ground plane under both feet and a horizon placed at the real eye level. Nothing about the lens matters: a wide phone and a long telephoto give the same answer from the same marks, which is why this works on photos you did not take.
Placing the horizon
The horizon is the camera's eye level, not the skyline and not the top of the far wall. The tests, in order of reliability: a flat sea or plain gives it directly; the eyes of other standing adults are on it (a standing photographer's eye level is close to theirs); the receding edges of a floor, a street or a ceiling meet on it, which the vanishing point finder will draw for you; and in a photo of a room, the edge where floor tiles stop foreshortening is a good guess. Drag the two ends of the dashed line separately if the camera was rolled; the tool reads the tilt and offers to level it.
A horizon placed too high makes everything read taller than it is and the camera height read high as well. If the camera height comes out at 2.5 m for a hand-held photo, the horizon is on the skyline and needs to come down to eye level.
Choosing a reference
Doors are the best reference in most photos: 2.0 m is the standard interior height across Europe and most of the world, 2.03 m (80 in) in the United States, and a front door may be taller. Other reliable heights: a standard storey is about 3 m floor to floor; a car roof is 1.4–1.5 m; a bus is 3.0–3.3 m; a kitchen counter is 0.9 m; an A4 sheet is 0.297 m tall for close-up work. A person is a fine reference if you know their height; an estimated one is not, since the result inherits the whole error.
The reference must stand on the same ground as the unknown. A door with a step, a car on a kerb, a person on a stage: each puts its base on a different plane and the fraction no longer means the same thing. Mark the base where the object touches the ground the unknown stands on.
What it will not do
It cannot measure a person against nothing. A portrait against a blank wall, or a selfie, has no known height in it and no horizon, so the geometry has nothing to anchor to. It also assumes both things stand upright on flat ground: a person sitting, leaning or on a slope, or a photo of a hill, will be wrong by the amount of the lean. A tilted camera needs the tilt option for the last percent: with the camera pitched 15° the level formula reads a 1.75 m person as 1.74 m, at 30° as 1.73 m, and the option removes that error entirely. For the numbers behind all of this, the figure height calculator runs the same relationship the other way, from a known height and distance to a size on the picture.
Elsewhere on this site
vanishing point finder · figure height calculator · figures in perspective · perspective in photography · horizon height calculator
Questions
How do you measure someone's height from a photo?
You need three things in the same photo: the horizon (the camera's eye level), one object of known height standing on the same ground, and the person. On any vertical object the horizon crosses it at exactly the camera's height, so the fraction of the object below the horizon is the camera height over the object's height. The reference object fixes the camera height; the same fraction on the person then gives theirs. This tool does that arithmetic from six dragged points.
How accurate is a height measured from a photo?
The arithmetic is exact; the error is in where you put the marks. On the sample photo (800 px tall), moving the horizon 10 px too high reads the 1.75 m person as 1.79 m, about 2 %, and every other object shifts the same way, which makes the horizon the mark to spend time on. A few pixels on the base or top of a 300 px object is about 1 %. A reference standing on different ground (a doorstep, a kerb) is the other common error, and shoes, hair and a lean add a few centimetres of their own.
Where is the horizon in a photo with no visible horizon?
At the camera's height. If the photographer was standing, look for the heads of other standing adults: the horizon passes through their eyes whether they are near or far. Indoors, it is where the receding lines of the floor and ceiling would meet; the vanishing point finder on this site draws it from two such lines. A flat sea or plain gives it directly.
Does the reference object have to be next to the person?
No. It can be anywhere in the photo as long as it stands on the same ground plane as the person and both bases are visible. A door at the far end of a corridor works for a person in the foreground. What it cannot be is something on a step, a table or a slope.
What if the camera was tilted up or down?
Tick "camera tilted" and drag the two yellow lines along vertical edges (door frames, corners of buildings). The tool finds where the verticals converge and uses the full cross-ratio formula instead of the level-camera one. A phone photo taken looking slightly down at someone is the usual case; without the correction it would read them a little short.
Can I use this for 'how tall do I look' photos?
Yes, if the photo has something of known height in it on the same ground, ideally a door or another person whose height you know. A photo of one person against a blank wall has nothing to measure against and no tool can do it.