Light Bulb & Lumens Calculator

Working out how many bulbs a room needs usually means hopping between three separate calculators: one for lumens, one for wattage, one for running cost. This tool covers all three in one form. Pick your room type, enter its area, and you'll get the lumens required, how many bulbs that comes to, wattage across LED, CFL, and incandescent, plus an estimated running cost.

Room & Lighting Details

Room / use case

Room area

Units

Lumens per bulb

A standard 800 lm bulb is roughly equivalent to an old 60W incandescent.

Bulb type (for wattage & cost)

Running cost (optional)

Currency

Cost per kWh

Hours used/day

Your lighting plan will appear here

Choose your room type and enter its area, then click calculate.

Quick Answer

Required lumens = your room's target lux × its area in square meters. A kitchen countertop needs a lot more light than a hallway, so this calculator has standard lux targets built in for common rooms, then divides that number by your bulb's lumen output to get a bulb count.

How It Works: Formula & Variables

Lumens Needed = Target Lux × Area (m²)

Square feet to square meters: sq m = sq ft × 0.092903
Bulbs needed: = lumens needed ÷ lumens per bulb, rounded up
Wattage equivalent: = lumens needed ÷ efficacy (lumens per watt)
Typical efficacy: LED ≈ 90 lm/W · CFL ≈ 60 lm/W · Incandescent ≈ 14 lm/W
Running cost: = (watts ÷ 1000) × hours/day × cost per kWh

Worked Examples

Example 1: Kitchen task lighting

A 120 sq ft kitchen (≈ 11.1 m²) needing 750 lux over the work area: 750 × 11.1 ≈ 8,325 lumens. At 800 lumens per bulb, that's 11 bulbs. In LEDs (90 lm/W) that's about 93W total; at $0.15/kWh for 4 hours a day, roughly $1.67/month.

Example 2: Bedroom ambient lighting

A 15 m² bedroom needing 100 lux: 100 × 15 = 1,500 lumens, or 2 bulbs at 800 lumens each. In incandescent bulbs (14 lm/W) that's about 107W, versus roughly 17W for the same light in LED.

Key Concepts

Lumens vs. lux: Lumens measure a bulb's total output; lux measures how much of that output actually reaches a surface a meter away, spread across an area. The same bulb lights a small room brightly and a large room dimly.

Bulb counts round up: Partial bulbs don't exist, so the bulb count always rounds up to the next whole number, meaning your actual light level usually ends up a bit above the bare minimum.

Efficacy is the real lever on cost: Switching from incandescent to LED can cut wattage, and running cost, by more than 80% for the same brightness.

Common Mistakes

Confusing lumens with watts: Watts measure power draw, not brightness, so a 60W incandescent and a 9W LED can put out the same lumens even though one uses far less electricity.

Using one lux target for the whole house: Task areas like counters, desks, and sewing tables need much brighter light than a hallway or living room, so treating every room the same leaves some spaces overlit and others underlit.

Forgetting real-world bulb spread: This calculator assumes light spreads evenly through the room, but fixture placement, lampshades, and ceiling height all affect how evenly it actually spreads in practice.

Frequently Asked Questions

Lumens measure the total light a bulb puts out; lux measures how much of that light actually lands on a surface. A bulb with plenty of lumens can still leave a big room dim, since those lumens spread over more area. That's why this calculator starts from a lux target per square meter instead of a flat lumens figure.

They're based on lighting guidelines commonly used by lighting designers and electricians: around 100 lux for a hallway, 300 lux for general kitchen lighting, 750 lux over a countertop or a sewing table where you need to see fine detail. Treat them as a good starting point rather than a rule, and use the custom lux option if you prefer things brighter or dimmer.

LEDs, CFLs, and incandescent bulbs turn electricity into light at very different rates. An LED can match an incandescent bulb's brightness using roughly a sixth of the power, so the calculator shows all three side by side and lets you choose which one feeds the cost estimate.

It's a plain estimate based on the wattage figure above, your hours of use per day, and your entered cost per kWh, so it's only as accurate as those numbers. It won't account for dimmers, motion sensors, or bulbs that don't run at full brightness the whole time, but it gives a reasonable ballpark for planning.

Yes. There's no fixed currency built in. Type in whatever symbol or code you use, dollars, euros, pounds, or a three-letter code, and it'll show up in your results.

Last reviewed 2026-07-08. For educational purposes only — not professional advice.