How far can a thermal camera see?
There's no fixed range. What changes with distance is how much wall each thermal pixel covers. Enter how far away you'll stand and what you want to see, and this calculator works it out for a 96×96 and a 256×192 sensor.
It depends on the size of what you're looking at. At 10 ft (3 m), one pixel of a 256×192 camera covers about 0.44 in (11 mm) of wall, and one pixel of a 96×96 camera about 1.2 in (30 mm). For a temperature reading you can trust, aim for at least 3 pixels across the thing you're checking.
Visible, but the reading blends with what's around it. Move closer
Clear: you can trust the temperature reading
One pixel, by distance
How much wall one thermal pixel covers, at a 50° horizontal field of view.
| Distance | 96×96 | 256×192 |
|---|---|---|
| 1 ft (0.3 m) | 0.12 in (3 mm) | 0.04 in (1.1 mm) |
| 3 ft (0.9 m) | 0.35 in (8.9 mm) | 0.13 in (3.3 mm) |
| 6 ft (1.8 m) | 0.7 in (18 mm) | 0.26 in (6.7 mm) |
| 10 ft (3 m) | 1.2 in (30 mm) | 0.44 in (11 mm) |
| 15 ft (4.6 m) | 1.7 in (44 mm) | 0.66 in (17 mm) |
| 20 ft (6.1 m) | 2.3 in (59 mm) | 0.87 in (22 mm) |
| 30 ft (9.1 m) | 3.5 in (89 mm) | 1.3 in (33 mm) |
Smallest thing you can read clearly
This is 3 pixels wide. Anything narrower can still show up in the picture, but its temperature blends with what's around it.
| Distance | 96×96 | 256×192 |
|---|---|---|
| 1 ft (0.3 m) | 0.35 in (8.9 mm) | 0.13 in (3.3 mm) |
| 3 ft (0.9 m) | 1 in (27 mm) | 0.39 in (10 mm) |
| 6 ft (1.8 m) | 2.1 in (53 mm) | 0.79 in (20 mm) |
| 10 ft (3 m) | 3.5 in (89 mm) | 1.3 in (33 mm) |
| 15 ft (4.6 m) | 5.2 in (133 mm) | 2 in (50 mm) |
| 20 ft (6.1 m) | 7 in (178 mm) | 2.6 in (67 mm) |
| 30 ft (9.1 m) | 10 in (266 mm) | 3.9 in (100 mm) |
How much the whole picture covers
Width × height of the area on screen. Spark's sensor is square; the maker gives one field of view for it, so we use 50° both ways.
| Distance | 96×96 | 256×192 |
|---|---|---|
| 1 ft (0.3 m) | 11 in × 11 in | 11 in × 8 in |
| 3 ft (0.9 m) | 2.8 ft × 2.8 ft | 2.8 ft × 2 ft |
| 6 ft (1.8 m) | 5.6 ft × 5.6 ft | 5.6 ft × 4.1 ft |
| 10 ft (3 m) | 9.3 ft × 9.3 ft | 9.3 ft × 6.8 ft |
| 15 ft (4.6 m) | 14 ft × 14 ft | 14 ft × 10 ft |
| 20 ft (6.1 m) | 19 ft × 19 ft | 19 ft × 14 ft |
| 30 ft (9.1 m) | 28 ft × 28 ft | 28 ft × 20 ft |
The formula
Size of one pixel = 2 × distance × tan(field of view ÷ 2) ÷ number of pixels across
Take Spark Pro at 3 m: 2 × 3 m × tan(25°) ÷ 256 = about 10.9 mm. Do the same for Spark at 3 m with 96 pixels across and you get about 29 mm. The calculator uses the horizontal field of view, because that's where the pixels are widest. You're welcome to use the formula or these tables; a link back to this page is appreciated.
Why 3 pixels?
One pixel gives a single temperature reading for everything in its patch of wall. If a pipe only fills part of that patch, the reading mixes the pipe with the wall beside it, so a hot pipe reads cooler than it really is. Three pixels across means the middle one sits fully on the target. This is a common rule of thumb in thermography rather than a maker's spec, and some pros ask for more when an exact number matters.
What the number doesn't tell you
- It's geometry, not a test result. It's worked out from the makers' sensor size and field of view. We didn't measure these distances.
- A target needs a temperature difference. A leak, a stud or a pipe only shows up when it makes the surface warmer or cooler than the rest. See how to read a thermal image.
- It doesn't see through walls. It reads the surface it's pointed at.
- Enhanced resolution adds no readings. The apps show Spark as 192×192 and Spark Pro as 512×384. That sharpens the picture, but the real readings still come from 96×96 and 256×192.
- Shiny surfaces mislead. Glass, mirrors and polished metal reflect heat from elsewhere.
Which sensor for which job?
| The job | Where you'll stand | Enough sensor |
|---|---|---|
| One outlet, a charger or a breaker | Within arm's reach | Spark or Spark Pro |
| A window frame or a patch of wall | About 3 ft (1 m) | Spark or Spark Pro |
| Studs and pipes across a whole wall or ceiling | About 10 ft (3 m) | Spark Pro |
More on the choice: Spark vs Spark Pro, and which phones they fit: thermal cameras for iPhone 15, 16, 17 and 18.
