Pixels Per Metre: The Number That Decides Whether Your CCTV Can Read a Plate

There's a moment every CCTV buyer has. You've unwrapped the camera, screwed it to the fascia, opened the app, and there on the screen is your own car looking gloriously sharp. Lovely. Then a van rolls past the gate and you squint at the playback and get... a pale rectangle with some grey smudges on it.

Welcome to the megapixel gap. The gap between how good the picture looks and how much of the picture is actually information.

What 4K really buys you

A 4K camera gives you roughly 8 megapixels and about 3,840 pixels across the width of the frame. That's four times the pixel count of a 1080p model. It sounds transformative. And it is — if your subject fills a decent chunk of the frame.

The problem is that a UK number plate is 520mm wide. Half a metre. That's it. Whether your camera is 2MP or 8MP, the plate is still half a metre wide, and the only thing that changes is how many of the camera's pixels get to sit on top of it.

The one calculation worth doing

Forget megapixels. Think in pixels per metre at the distance you care about.

Here's the rough method. Take the horizontal pixel count of the camera, and divide it by the width of the scene it covers at your target distance. To get that width, you either check the manufacturer's spec sheet (look for horizontal field of view) or just measure it — stand where the car will be, look at the app, and note what's visible edge to edge.

A worked example. A 4K camera with a fairly typical 90° horizontal lens, looking at a spot 15 metres away, covers a scene around 30 metres wide. That's 3,840 ÷ 30 = 128 pixels per metre. A 520mm plate therefore gets about 66 pixels across it.

Sixty-six pixels is not a registration. It's an impression of one.

For anything approaching reliable reading, you want roughly 200 pixels across the plate. That's the ballpark UK automatic number plate recognition installs work to. So on that same camera, you need to be within about 5 to 7 metres of the plate to have a fighting chance — and in practice you'll want closer still.

What that means for real drives

A typical UK driveway is short — a car length or two, maybe 6 to 10 metres from the front door to the pavement. That sounds encouraging until you remember the camera is up on the house, pointing diagonally, and the interesting bit (a stranger's vehicle at the gate, a delivery van on the road) is at the far end of that diagonal, not the near end.

Rough guide, assuming a decent 4K camera with a moderate lens:

• Plate at 3–5m: comfortably readable in good light. This is the sweet spot.
• Plate at 8–10m: readable on a good day, marginal at night, dependent on shutter and lighting.
• Plate at 15m+: you're capturing a shape, not a registration. No amount of firmware will save you.

If you genuinely need plates read at 15 or 20 metres, you don't want a general-purpose security camera. You want a dedicated ANPR unit with a long lens and a narrow field of view — accepting that it will see almost nothing else.

Angle: the quiet killer

A camera looking straight down a drive at a plate face-on works far better than one looking across it at 45 degrees. Angle does two things: it foreshortens the plate (so fewer pixels land on it), and it introduces perspective distortion that makes characters harder to distinguish.

Aim for the shallowest angle you can manage — ideally within about 30 degrees of straight-on. And mount low enough that you're not looking down at a steep rake, which flattens the plate into an ellipse.

Shutter speed and the blur you can't undo

Most domestic cameras default to a slow shutter so the picture looks bright and clean at night. That's great for spotting a fox. It's fatal for a moving vehicle.

A car doing 20mph covers about 9 metres every second. With a slow exposure, the plate smears across the sensor and no amount of sharpening in the app will recover it. You want the option to push the shutter to around 1/500s or faster for capture, and you'll need to accept a darker image as the trade-off — which means lighting becomes your friend.

If your camera has no manual shutter control anywhere in the settings, that tells you something about who it was designed for.

Night, infrared, and the retroreflective problem

UK plates are retroreflective by design — they throw light back at the source. That's brilliant for ANPR cameras with dedicated IR illuminators at the right angle. It's a nightmare for a general camera whose IR LEDs sit centimetres from the lens, because the plate bounces the IR straight back and blows out into a white slab.

You'll also fight IR wash on wet tarmac, spider webs across the lens, and headlight flare. All of which is to say: if number plate capture at night matters to you, plan for visible or dedicated infrared illumination positioned away from the camera axis, and test it before you commit.

Compression: the bit that eats detail

Two cameras with identical sensors can produce wildly different plates depending on bitrate. Aggressive H.265 compression smooths away exactly the high-frequency detail that makes a 7 look like a 1. If your camera lets you raise the bitrate, do it — and drop the frame rate to 15fps if you need to balance storage. For plates, resolution and bitrate beat smooth motion every time.

Three tests you can run this weekend

1. The lawn-chair test. Put a spare plate, or a printout of one at true size, on a chair at the far end of your drive. Look at it live in daylight, then at dusk, then with the IR on. Can you read it? If not, the camera won't either.
2. The rolling test. Have someone walk a plate past at jogging pace, then at a slow drive. Check playback frame by frame. Blur shows up here long before it shows up in a live view.
3. The honest-distance test. Measure how far your camera actually is, diagonally, from where a vehicle would pause. Most people overestimate their coverage by a factor of two.

What to buy instead of more megapixels

When you're comparing models, ignore the badge on the box and look for these instead:

• Sensor size and pixel size, not just resolution. A good 4MP sensor with larger pixels will beat an 8MP with tiny ones once the sun goes down.
• A lens you can adjust. Varifocal beats fixed if you need to trade field of view for detail.
• Manual shutter and gain control. Non-negotiable if vehicle capture is the point.
• Realistic bitrate options and proper WDR for backlit gates.

A well-placed 4MP camera at 4 metres will out-read a badly placed 8MP camera at 14 metres every single time. Position beats pixels. It always has.