Photometry hub: lumens, lux, foot-candles, candela, nits, lumens/watt -- plus watts-to-lumens by bulb type, a room lighting planner, inverse-square-law and beam-angle calculators.
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1. Photometry Unit Hub
Convert any lighting unit to any other within the same quantity -- luminous flux, intensity, illuminance, or luminance. For conversions that need context (area, distance, beam angle), use the calculators below.
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enter a value and convert
All units at a glance
Unit
Symbol
Quantity
Value
Lux to lumens with room area
Illuminance and flux are linked by area: lux = lumens / m2.
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Candela to illuminance at a distance
Inverse square law: E = I / d2 -- lux if distance is in meters, foot-candles if in feet.
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Candela to lumens via beam angle
Total flux inside the beam: lumens = candela x 2pi(1 - cos(angle/2)).
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2. Watts to Lumens by Bulb Type
Watts measure power; lumens measure brightness. Multiply by the lamp's efficacy (lumens per watt) to translate between them. Efficacy values below are typical -- the table shows the usual range.
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Luminous efficacy reference table
Lamp technology
Typical efficacy
Usual range
Incandescent
15 lm/W
12-17 lm/W
Halogen
20 lm/W
16-24 lm/W
CFL (compact fluorescent)
60 lm/W
50-70 lm/W
Linear fluorescent (T8/T5)
90 lm/W
80-100 lm/W
LED
100 lm/W
80-120 lm/W
Metal halide
95 lm/W
75-100 lm/W
High-pressure sodium
120 lm/W
100-150 lm/W
Honest note: efficacy varies by brand, color temperature, and age. The calculator uses the typical value; treat results as estimates within about +/-20%.
Common bulb equivalences
Incandescent
Light output
Halogen equiv.
CFL equiv.
LED equiv.
40 W
~450 lm
~28 W
~9 W
~5 W
60 W
~800-900 lm
~42 W
~13-15 W
~8-10 W
75 W
~1,100 lm
~53 W
~18 W
~11-12 W
100 W
~1,600 lm
~70 W
~23-26 W
~15-17 W
150 W
~2,600 lm
~105 W
~40 W
~25 W
About this table: these are real-world manufacturer equivalence ratings (a "60 W equivalent" LED is sold as ~800 lm). They sit slightly below the 15 lm/W typical value used by the calculator above -- both are honest estimates within normal product variation.
3. Room Lighting Planner
How many lumens does a room need? Multiply the floor area by the target illuminance for the room type: lumens = lux target x area (m2).
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Recommended illuminance by room type
Room / space
Target lux
Target foot-candles
Lumens per m2
Hallway / corridor
100
~9.3
100
Bedroom
150
~13.9
150
Living room
150
~13.9
150
Dining room
200
~18.6
200
Warehouse
200
~18.6
200
Kitchen
300
~27.9
300
Bathroom
300
~27.9
300
Classroom
300
~27.9
300
Home office desk
500
~46.5
500
Office
500
~46.5
500
Retail
500
~46.5
500
Workshop / garage
750
~69.7
750
Design note: these are maintained illuminance targets from common lighting practice. Dark walls, high ceilings, and aging lamps all reduce delivered lux -- add 20-30% headroom for real rooms.
4. Beam Angle and Distance
Two tools for aiming light: how illuminance changes with distance (inverse square law), and how wide a beam spreads (coverage diameter).
Illuminance at a new distance
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E2 = E1 x (d1/d2)2 -- double the distance, quarter the light
Beam angle to coverage diameter
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diameter = 2 x distance x tan(angle / 2)
5. Reference Tables
Exact conversion factors between every supported unit. "To base" means the factor you multiply by to reach the base unit of that quantity.
Luminous flux (base: lumen)
Unit
Symbol
To lumens
Lumen
lm
1
Kilolumen
klm
1,000
Millilumen
mlm
0.001
Luminous intensity (base: candela)
Unit
Symbol
To candela
Candela
cd
1
Kilocandela
kcd
1,000
Candlepower
cp
1 (historical equivalence)
Illuminance (base: lux)
Unit
Symbol
To lux
Lux
lx
1
Kilolux
klx
1,000
Millilux
mlx
0.001
Foot-candle
fc
10.7639104167
Phot
ph
10,000
Luminance (base: nit = cd/m2)
Unit
Symbol
To nits
Nit
nt
1
Stilb
sb
10,000
Foot-lambert
fL
3.4262590996
Lambert
L
3,183.0988618
Apostilb
asb
0.3183098862
Light energy (base: talbot)
Unit
Symbol
To talbot
Talbot (lumen-second)
T
1
Lumen-second
lm.s
1
6. Worked Examples
Example 1 -- LED swap: a 60 W incandescent at 15 lm/W gives 60 x 15 = 900 lumens. An LED at 100 lm/W needs 900 / 100 = 9 W for the same brightness.
Example 2 -- room lighting: a 20 m2 kitchen at the 300 lux target needs 300 x 20 = 6,000 lumens. With 800-lumen bulbs that is 6,000 / 800 = 7.5, rounded up to 8 fixtures.
Example 3 -- foot-candles to lux: a US spec sheet calls for 100 foot-candles. 100 x 10.7639104167 = 1,076.39 lux. Going the other way, 500 lux / 10.7639104167 = 46.45 foot-candles.
Example 4 -- beam coverage: a 60-degree beam aimed at a wall 3 m away covers 2 x 3 x tan(30deg) = 3.46 m in diameter. A 1,000 cd spotlight with that 60-degree beam emits 1,000 x 2pi(1 - cos 30deg) = 841.79 lumens within the beam.
Example 5 -- inverse square: a lamp reads 500 lux at 2 m. At 4 m the illuminance is 500 x (2/4)2 = 500 / 4 = 125 lux.
7. Methodology
Definitions
Luminous flux (lumens)
total visible light emitted by a source
Luminous intensity (candela)
flux per unit solid angle, in one direction
Illuminance (lux / foot-candle)
flux landing on a surface: 1 lux = 1 lm/m2, 1 fc = 1 lm/ft2
Luminance (nit)
brightness of a surface as seen: 1 nit = 1 cd/m2
Luminous efficacy (lm/W)
how efficiently power becomes visible light
Formulas used
lux = lumens / area (m2)
E = I / d2 (inverse square law)
beam diameter = 2 x d x tan(angle / 2)
lumens in beam = candela x 2pi(1 - cos(angle / 2))
lumens = watts x efficacy (lm/W)
Scope boundary
This tool is photometry only -- lumens, lux, candela, foot-candles, nits, and lamp efficacy. It is intentionally separate from our power unit converter (#282/#284 family: volts, amps, watts of electrical power, horsepower, BTU/h, dBm), because lighting units and electrical units answer different questions. Lamp wattage appears here solely to estimate brightness, never as an electrical quantity.
Honesty notes
Efficacy and room-lux targets are typical values, not guarantees -- real products vary by brand and color temperature. The foot-candle factor 10.7639104167 is exact (1 / 0.09290304). Inverse square law assumes a point source in free space; reflectors and room surfaces change real results.
8. Frequently Asked Questions
How many lumens do I need per square foot?
One foot-candle equals one lumen per square foot, so lumens per square foot is simply your target foot-candle level. Typical values: living room and bedroom about 15 lumens per square foot (150 lux), kitchen and bathroom about 30 (300 lux), home office about 50 (500 lux). Multiply by the room's area in square feet for the total lumens needed.
How do I convert lux to lumens?
Multiply the lux value by the area in square meters: lumens = lux × m². Lux is lumens per square meter, so 500 lux spread over 20 m² equals 10,000 lumens. If your area is in square feet, convert it to square meters first (1 ft² = 0.09290304 m²), or use the context converter above which handles both.
How many lumens is a 60-watt bulb?
A 60 W incandescent bulb produces about 900 lumens (60 × 15 lm/W). The LED equivalent is roughly 8–10 W (900 ÷ 100 lm/W ≈ 9 W). Watts measure electrical power while lumens measure brightness, so always compare lumens when swapping technologies — never watts.
What is the difference between lumens and lux?
Lumens measure the total light a source emits (luminous flux); lux measures how much of that light lands on a surface (illuminance = lumens ÷ m²). A 1,000-lumen bulb gives 100 lux over 10 m², or 1,000 lux over 1 m². Lumens describe the bulb; lux describes the brightness you experience at a surface.
How many foot-candles are in one lux?
One foot-candle equals 10.7639 lux, exactly. To convert lux to foot-candles, divide by 10.7639 — for example 500 lux ≈ 46.45 foot-candles. The US lighting industry traditionally uses foot-candles while most of the world uses lux; the converter above translates both ways.
What is the difference between candela and lumens?
Candela measures luminous intensity in one direction; lumens measure total output in all directions. They are linked by the solid angle: lumens = candela × steradians. For example, a 1,000-cd spotlight with a 60° beam emits about 842 lumens inside that beam — the same lumens in a narrower beam means a higher candela.
What is a nit in display brightness?
A nit is one candela per square meter (cd/m²), the unit of screen luminance. Typical values: a laptop screen around 300 nits, an HDR television 1,000+ nits, and sunlight-readable outdoor displays above 2,000 nits. The converter's luminance tab translates nits to stilb, foot-lamberts, and lamberts.
What is the inverse square law for light?
Illuminance falls with the square of the distance from the source: E = I ÷ d². Double the distance and you get one quarter of the light — for instance 500 lux at 2 m becomes 125 lux at 4 m. This applies to point sources in free space; use the beam & distance tab above to calculate it.
How many lumens do I need for a room?
Multiply the room's area by the target illuminance: lumens = lux × m². A 20 m² kitchen at 300 lux needs 6,000 lumens — roughly eight 800-lumen bulbs. Pick your room type in the room planner above and it applies the recommended lux level automatically.
How do I convert watts to lumens?
Multiply watts by the lamp's efficacy in lumens per watt: LED ≈ 100 lm/W, CFL ≈ 60, halogen ≈ 20, incandescent ≈ 15. So a 9 W LED gives roughly 900 lumens — the same brightness as a 60 W incandescent at one-seventh of the power. The watts-to-lumens tab above includes the full efficacy table.
What does lumens per watt mean?
Lumens per watt (lm/W) is luminous efficacy — how efficiently a lamp converts electricity into visible light. Higher is better: incandescent about 15, halogen about 20, CFL about 60, LED about 100 lm/W. It is the fair way to compare lighting technologies, and the theoretical maximum is 683 lm/W.
What is beam angle and how do I calculate coverage?
Beam angle is the cone of light leaving a fixture, measured in degrees. The coverage diameter at a given distance is 2 × distance × tan(angle ÷ 2) — for example a 60° beam at 3 m covers about 3.46 m across. A narrower beam concentrates the same lumens into a smaller area, raising the lux.
What lux level do I need for an office or kitchen?
Typical design values: office desk work 500 lux, kitchen 300 lux, bathroom 300 lux, classroom 300 lux, living room and bedroom 150 lux, hallways 100 lux, workshops 750 lux. These are maintained illuminance targets from common lighting practice — the room planner above lists all of them.
Is this the same as an electrical power unit converter?
No. This tool covers photometry — lumens, lux, candela, foot-candles — which is the science of visible light. For electrical power units such as watts, horsepower, BTU/h, and dBm, use our separate power unit converter. The only overlap is lamp wattage, used here purely to estimate brightness.
How bright is 1,000 lumens in real terms?
About as bright as an old 75 W incandescent bulb. It delivers 100 lux over 10 m² — a small room at comfortable living-room brightness — or 1,000 lux over 1 m², which is bright task lighting. Most living rooms need 3,000–6,000 lumens in total across all fixtures.
Why does doubling the distance quarter the brightness?
Light from a point source spreads over a sphere whose area grows with the square of the radius (4πr²). Twice the distance means four times the area, so each square meter receives one quarter of the light. That is the physical reason behind the inverse square law, E = I ÷ d².
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