Lumens vs Lux Explained: How to Calculate the Right Lighting for Every Room

Choosing the right lighting for your home is more than just picking a bulb that fits the socket. Two terms you will encounter when shopping for LED bulbs and fittings are lumens and lux — and understanding the difference can transform how well-lit and comfortable your rooms feel. This guide explains what they mean and, crucially, how to calculate exactly how much light you need for any room in your home.

What Are Lumens?

A lumen (lm) is the unit used to measure the total amount of visible light emitted by a source. In simple terms: the higher the lumens, the brighter the light.

Gone are the days when we bought bulbs by wattage. Watts measure energy consumption, not brightness. With LED technology, a 9W LED bulb can produce the same lumens as an old 60W incandescent. Always check the lumen output on the packaging — not the wattage.

Quick reference — old watts vs lumens:

• 40W incandescent ≈ 450 lumens
• 60W incandescent ≈ 800 lumens
• 100W incandescent ≈ 1,600 lumens

What Is Lux?

Lux (lx) measures how much light actually falls on a surface. It takes distance into account: the same bulb will produce fewer lux on a surface far away than on one close up.

The relationship is:

1 lux = 1 lumen per square metre

This is why lux is the more practical measurement when planning room lighting — it tells you how bright a surface will actually appear, not just how much light the bulb produces in total.

Recommended Lux Levels by Room

Different activities require different light levels. Here are the UK-standard recommended lux levels for domestic rooms:

• Bedroom: 100–300 lux (general), 500 lux at a dressing table or reading area
• Living Room: 150–300 lux (general), 400–500 lux for reading
• Kitchen (general): 300 lux
• Kitchen (worktop/task area): 500–750 lux
• Bathroom: 300 lux (general), 500 lux at the mirror
• Home Office: 500 lux
• Hallway / Landing: 100–150 lux

How to Calculate Lumens for a Room

Here is the simple formula:

Total Lumens Needed = Room Area (m²) × Required Lux Level

Step-by-Step Example: Kitchen

Let us say your kitchen is 4m × 3m = 12m² and you want a general light level of 300 lux, with brighter task lighting over the worktops at 500 lux.

General lighting:

• 12m² × 300 lux = 3,600 lumens total
• If using 600 lumen GU10 downlights, you need approximately 6 downlights

Worktop task lighting (assuming 3m of worktop):

• Approximately 3m² worktop area × 500 lux = 1,500 lumens for under-cabinet LED strips

Step-by-Step Example: Bedroom

A typical bedroom measuring 4m × 4m = 16m², targeting 150 lux for relaxed general lighting:

• 16m² × 150 lux = 2,400 lumens total
• Using 400 lumen GU10 bulbs: you need approximately 6 bulbs
• Add a bedside lamp at 300–500 lumens for reading

Practical Tips for Getting It Right

Use Multiple Light Sources

Rather than relying on a single central ceiling light, layer your lighting. Combine ceiling downlights for general illumination with under-cabinet strips for task lighting and lamps or wall lights for ambience. This gives you flexibility for different moods and activities.

Account for Room Finish

Dark walls and ceilings absorb more light than pale ones. If your room has dark décor, increase your lumen calculation by 20–30% to compensate.

Ceiling Height Matters

The higher the ceiling, the more lumens you need to achieve the same lux at floor level. For ceilings above 2.7m, increase your total lumen estimate by 10–15% per additional 30cm of height.

Dimmable Circuits

Installing dimmable LED downlights means you can run the circuit at full brightness for task lighting and dial it back for evenings. This makes it easy to hit both the higher and lower ends of the recommended lux range from the same fittings.

Quick Lumen Calculator Reference Table

Use this table as a starting point — adjust for room finish and ceiling height:

• Small bedroom (10m²) @ 150 lux = 1,500 lm
• Medium bedroom (15m²) @ 150 lux = 2,250 lm
• Large bedroom (20m²) @ 200 lux = 4,000 lm
• Small kitchen (8m²) @ 300 lux = 2,400 lm
• Medium kitchen (12m²) @ 300 lux = 3,600 lm
• Large kitchen (20m²) @ 300 lux = 6,000 lm
• Living room (18m²) @ 200 lux = 3,600 lm
• Bathroom (6m²) @ 300 lux = 1,800 lm
• Home office (10m²) @ 500 lux = 5,000 lm

Lighting Angles: Why Beam Angle Matters

The beam angle of a downlight or spotlight determines how wide or narrow the cone of light is. Two bulbs with identical lumen output can produce very different results in a room depending on their beam angle — and getting this wrong is one of the most common lighting mistakes.

Understanding Beam Angles

• Narrow Spot (15°–25°) — Concentrates light into a tight beam. Ideal for highlighting artwork, shelves, or architectural features. Creates dramatic contrast but leaves surrounding areas dim.
• Spot (25°–40°) — A focused beam with a little more spread. Good for accent lighting and task areas like kitchen worktops.
• Wide Flood (40°–60°) — The most popular choice for general domestic lighting. Provides good coverage from a standard ceiling height without harsh hot spots.
• Very Wide / Diffuse (60°–120°) — Spreads light broadly across a wide area. Best for low ceilings or large open-plan spaces where maximum coverage from fewer fittings is needed.

Beam Angle vs Ceiling Height

The higher your ceiling, the wider the beam angle you need to maintain adequate lux at floor level. As a general guide:

• 2.4m ceiling — 60° beam angle works well for general lighting
• 2.7m ceiling — 60°–90° recommended for even coverage
• 3m+ ceiling — 90°–120° or increase the number of fittings

Calculating Coverage Per Downlight

You can estimate the diameter of the lit area on the floor using this formula:

Coverage diameter = 2 × ceiling height × tan(beam angle ÷ 2)

For example, a 60° downlight at a 2.4m ceiling height:

• 2 × 2.4 × tan(30°) = 2 × 2.4 × 0.577 = 2.77m diameter

This means each fitting covers roughly a 2.8m circle at floor level. For a 4m × 3m kitchen, you would need at least 4–6 fittings for even coverage.

Spacing Your Downlights

A common rule of thumb: space downlights at a distance equal to half their floor coverage diameter. So for a 60° fitting at 2.4m height (2.8m coverage), space fittings approximately 1.2–1.4m apart for overlap and even illumination without dark patches.

Also keep downlights at least 0.5m away from walls to avoid scalloping — those crescent-shaped shadows that make walls look uneven.

Practical Room Recommendations

• Kitchen general lighting: 60° wide flood for even worktop coverage
• Kitchen under-cabinet task lighting: 30°–40° spot to direct light onto the worktop surface
• Bedroom: 60° wide flood for ambient; 25°–40° spot for reading light over the bed
• Living room general: 60° wide flood; 25° spot for picture lights or accent features
• Bathroom mirror: 40°–60° to illuminate the face evenly without glare
• Hallway: 90°–120° to maximise spread from fewer fittings

Choosing the Right Bulbs and Fittings

Once you know your total lumen requirement, divide it by the lumen output of your chosen bulb to get the number of fittings you need.

APM Plumbing & Electrical stocks a wide range of LED lighting options to suit every room and budget — from GU10 LED bulbs for existing spotlight fittings to fire-rated LED downlights for new installations, and LED panel lights for kitchens and offices. All our LED products display clear lumen output ratings so you can plan with confidence.

Summary

• Lumens = total light output from a bulb
• Lux = lumens per square metre (how bright a surface actually is)
• Formula: Room m² × target lux = total lumens needed
• Kitchens need 300 lux general, 500–750 lux on worktops
• Bedrooms need 150–300 lux general, more for reading areas
• Always choose dimmable LEDs for flexibility

Need help choosing the right fittings for your project? Browse APM's full LED lighting range or get in touch with our team for expert advice.

Visit APM Electricals at 24 Western Avenue, Acton, London W3 7TZ or call 020 8702 8080. Trade counter open 6 days a week — same-day collection on most stock.