How to calculate Lux for home

Every space in your home matters, and getting the right amount of light in each one matters just as much. Before you start distributing fixtures around the house, you need to study the space first.

The good news: lux calculation is easy. The bad news: there are a few terms you need to understand, and some room-by-room work to be done. Once you have both, the rest is just math.

Lumen vs. Lux: the two terms you actually need

Let's start with lumen. Lumen is the amount of light a fixture can produce.

Here's the part most people get wrong: we don't measure light output in watts. Watt is an old term, and honestly, why we still use it today is a story for another day. What you need to know right now is that watt only measures the electrical consumption of a device, in our case, the fixture. It tells you how much power the fixture is drawing, not how much light it's giving you. Lumen is the number that actually tells you that.

Now, the light coming out of a fixture doesn't stay in one place. It spreads across the room. Some of it lands on your dining table, some on the floor, some on the wall. The amount of that light actually hitting a surface is what we call lux.

So: lumen is how much light a fixture produces. Lux is how much of that light is landing on a given surface.

1 lux = 1 lumen per square meter.

Why this matters: the standards

Once you understand lumen and lux, you can understand why experts set standards for how much lux each type of space needs, and that standard depends entirely on what the room is for.

A bedroom doesn't need much: 150 lux is enough for a space meant for rest. A hallway is a walking area, not a working one, so 50 lux is enough to see where you're going. An office, where reading and task work happen, needs 300 lux. A technical drafting or engineering office, where precision matters, can go as high as 2,000 lux.

Here's the part people miss in both directions: going under the standard hurts your eyes when you're trying to focus, and going over it wastes money, on both the electricity and the fixtures themselves.

Recommended lux levels by space

Sourcing note: there isn't one single ISO standard that covers every room in a house. Task-oriented spaces (kitchen counters, home offices) are generally referenced against EN 12464-1 / ISO 8995-1 (lighting of work places), while general living spaces (living rooms, bedrooms) follow IES (Illuminating Engineering Society) residential benchmarks. Both are considered industry standard. Confirm this framing is acceptable before publishing.

How to calculate: the formula

Now that you know the terms, all we need is a simple formula:

Total lumens needed in the room = Standard lux × room area

There's another factor to consider here: the light coming out of a fixture doesn't all land on the surface. Some of it gets lost along the way: factors like ceiling height, wall color, and how the fixture is installed all reduce the lumen amount that actually reaches your surface. Calculating this precisely takes a lot of work, but we can set some general rules to narrow down the margin of error:

• Standard ceiling height (around 3m), downlights: you won't lose much, but to stay on the safe side, reduce the lumen output by 20% in your calculation.

• Cove lighting: since it's installed in a hidden cove and needs to reflect off the ceiling before it reaches your surface, it typically loses 30–40%. To stay safe, we calculate it as a 50% loss.

Now, back to the standard lux: this part's easy. Just decide what the room's function is. For a living room, we already picked our standard: 150 lux. The space is just as simple: multiply length by width.

Example: A living room, 4m wide by 6m long. What's the lumen requirement?

Lumens = 150 × (4 × 6) = 3,600 lumens

Now, 3,600 lumens is the exact amount we need, but we're going to pump it up by 30%. Why? LEDs lose lumen output over time. The minute you turn the light on, it starts losing lumens. For example, an LED rated L70 at 50,000 hours loses 30% of its light output by the time it hits 50,000 hours of use. Building in that 30% now means the room still has enough light once the fixtures have aged, not just on day one.

So the magic number isn't 3,600. It's 3,600 × 130%. Now we need to decide what light sources are actually emitting light in the space, usually downlights and cove lights, if you have them.

One thing to note: some fixtures don't count in this calculation. Chandeliers, wall lights, and decorative lighting are just decorative. They aren't part of it.

Copy this into Excel

Paste this into Excel. The formula cells will calculate automatically. Drag it down and swap in your own lux standard, dimensions, fixture lumens, and loss factor, room by room.

and you can download it here

That's the whole method: know your terms, pick your standard, do the math, then decide your fixtures. Skip a step and you either end up straining your eyes every evening, or paying for fixtures and lumens you'll never use. Do this and you'll notice your fixture count comes down, while you're still sure you're getting the right amount of light for the space.