The luminous flux converter above moves a light output value between lumens and the related units used in lighting specifications, photometry and older lamp catalogues. Enter a figure, choose two units, and the result appears immediately, with the same quantity shown at once in lumens, kilolumens, spherical candlepower and the equivalent radiant watts at the peak of human visual sensitivity.
Arb Digital publishes this alongside a wider set of free measurement tools. Luminous flux is the number printed on every lamp box sold today, replacing the wattage figure that used to stand in for brightness, and it is routinely confused with three other photometric quantities that sound similar. Getting the distinction right is most of the value of this page.
What This Luminous Flux Converter Does
Luminous flux is the total amount of visible light a source emits in all directions, weighted by how sensitive the human eye is to each wavelength. Its SI unit is the lumen. A lamp emitting 800 lumens is emitting that total regardless of whether the light is spread evenly around a room or focused into a narrow beam.
That weighting matters. Luminous flux is not raw radiant power. Two lamps radiating identical watts of electromagnetic energy will have very different lumen ratings if one emits mostly deep red and the other emits mostly green, because the eye's response peaks near 555 nanometres and falls away sharply toward both ends of the visible band. The lumen is deliberately a physiological unit, not a purely physical one, which is why converting lumens to watts requires knowing the spectrum.
The converter covers the SI prefixes, the definitional candela-steradian, spherical candlepower from older lamp literature, and the radiant-watt equivalent at 555 nm. Every result is shown live, and the grid gives you four common expressions of the same quantity so you can cross-check a specification sheet without running the tool four times.
Luminous Flux Is Not Luminous Intensity
This is the distinction that causes the most trouble, so it deserves its own section. Luminous flux, measured in lumens, is total light output summed over every direction. Luminous intensity, measured in candelas, is light output per unit solid angle — that is, brightness in one particular direction. They are related but they are not interchangeable, and no single number converts one to the other without knowing how the light is distributed.
The link is the steradian, the SI unit of solid angle. One lumen is one candela emitted into one steradian. A full sphere contains 4π steradians, approximately 12.566, so a source radiating uniformly in every direction at one candela emits 4π lumens — about 12.57 lm. That is exactly what the old term "spherical candlepower" meant, and it is why this converter offers it as a unit.
The catch is the word uniformly. Real sources are not uniform. A spotlight concentrating its output into a narrow cone can have an enormous candela figure and a modest lumen figure, because it puts a lot of light into very few steradians. A bare frosted bulb does the opposite. If you need the candela side of this relationship, use our luminous intensity converter, which handles candelas and their historical equivalents. This page deliberately restricts itself to flux so that no cross-quantity conversion is possible from the unit list — converting lumens to candelas requires beam geometry that a unit converter cannot assume.
How to Use It
- Enter the luminous flux value from a lamp specification, photometric report, or datasheet.
- Choose the source unit — lumens for anything modern, candlepower for older catalogues.
- Choose the target unit you need.
- Read the result, which updates as you type; the Convert button remains for keyboard and assistive-technology users.
- Use Swap units to reverse the direction of the conversion.
The Formula: How Luminous Flux Conversion Is Calculated
All conversions pivot through the lumen:
result = value × factor(from) ÷ factor(to)
Two factors here are worth verifying against the source. First, the candela-steradian equals exactly one lumen, because that is the definition of the lumen as a coherent derived unit in the BIPM SI Brochure — no rounding is involved. Second, the watt-at-555-nm factor is 683 lumens per watt exactly. This is not an approximation either: since the 2019 revision, the candela is defined by fixing the luminous efficacy of monochromatic radiation at a frequency of 540 × 10¹² hertz to be exactly 683 lumens per watt. That frequency corresponds to about 555 nanometres in air, the peak of the standard photopic response curve.
The spherical candlepower factor is 4π, approximately 12.566370614, following the same solid-angle geometry documented in NIST Special Publication 811. The SI prefixes are exact powers of ten as usual.
Reference Table: Every Supported Unit
Each factor is the number of lumens in one of that unit.
- lumen (lm) — 1 (SI base for this quantity)
- kilolumen (klm) — 1 × 10³ lm
- megalumen (Mlm) — 1 × 10⁶ lm
- millilumen (mlm) — 1 × 10⁻³ lm
- microlumen (µlm) — 1 × 10⁻⁶ lm
- candela-steradian (cd·sr) — 1 lm exactly; the definition written out
- spherical candlepower — 4π lm ≈ 12.566370614 lm
- watt at 555 nm (radiant) — 683 lm exactly, by the definition of the candela
Lumens, Lux and the Difference a Room Makes
Lumens describe the lamp. Lux describes the surface. One lux is one lumen spread over one square metre, so the illuminance a room receives depends on both the flux emitted and the area it lands on. A 1,000-lumen fitting produces 100 lux across ten square metres and only 10 lux across a hundred. This is why lighting designers specify lux for workspaces and lumens for products: the first is a requirement, the second is a component rating.
The practical consequence is that comparing two lamps by lumens alone tells you nothing about whether either will light a given space adequately. You also need the beam distribution, the mounting height, the surface reflectances and the fitting's optical efficiency, which is why photometric files exist. Converting lumens between units is exact arithmetic; predicting lux on a desk is a modelling exercise. To move between the area units involved in that calculation, our area converter handles square metres and square feet.
Why Lumens Per Watt Replaced Wattage on Lamp Boxes
For most of the twentieth century, wattage was a reliable proxy for brightness because almost every household lamp used the same technology. A 100 W incandescent was brighter than a 60 W incandescent because both converted electrical power to light with similar inefficiency. Once fluorescent and then LED sources arrived, that proxy collapsed: an LED delivering the same light as a 60 W incandescent draws under 10 watts.
Regulators responded by requiring luminous flux on packaging, so buyers compare light output directly rather than power consumption. The derived figure that matters for efficiency is luminous efficacy, in lumens per watt of electrical input. Incandescent lamps managed roughly 15 lm/W. Modern general-service LEDs commonly reach well over 100 lm/W. Note that this electrical efficacy is a different number from the 683 lm/W definitional constant, which describes an ideal monochromatic source at the eye's peak sensitivity and represents the absolute physical ceiling for any light source at that wavelength. For the electrical side of an efficacy calculation, see the power converter.
Reading Lumen Ratings Honestly
Two lumen figures appear on lighting datasheets and they are not the same. Source lumens, sometimes called raw or LED lumens, describe what the emitters produce. Delivered lumens, or fixture lumens, describe what actually leaves the finished product after losses in the reflector, lens, diffuser and thermal derating. The gap can easily be twenty percent or more, and a torch advertised on source lumens will not measure up against one rated on delivered output.
Thermal behaviour adds another layer. LED output falls as junction temperature rises, so a measurement taken at switch-on is higher than the steady-state figure a few minutes later. Standardised test methods exist precisely to stop that ambiguity, requiring measurement after thermal stabilisation. When you convert a lumen value, you are converting whichever of these numbers you were given — the arithmetic is exact, but the input carries a test condition with it, and comparing a source-lumen figure with a delivered-lumen figure produces a conclusion that is wrong no matter how carefully the units are handled.
Arb Digital builds product and specification pages that present photometric data clearly and are structured so search engines surface the right spec to the right buyer.
Web Development Services Browse All Free ToolsCommon Mistakes to Avoid
- Treating lumens and candelas as the same quantity — flux is total output, intensity is output per steradian.
- Converting lumens to candelas with a single factor — that only works for a perfectly uniform source, where the factor is 4π.
- Confusing the 683 lm/W definitional constant with a lamp's efficacy — the first is a physical ceiling at one wavelength, the second is a real product measurement.
- Comparing source lumens with delivered lumens across two products, which flatters whichever one is quoted raw.
- Assuming lumens predict lux without accounting for area, beam distribution and mounting height.
Related Free Tools From Arb Digital
Use the luminous intensity converter for candelas and directional brightness, the area converter when working out illuminance over a surface, and the power converter for the electrical watts behind an efficacy figure. The energy converter covers kilowatt-hours for running-cost comparisons, and the frequency converter is useful when working from the 540 terahertz definition. Everything else is in the free online tools hub.
Frequently Asked Questions
Lumens measure total luminous flux emitted in all directions. Candelas measure luminous intensity, which is flux per unit solid angle in one specific direction. A narrow beam can have high candelas and modest lumens.
Only for a source radiating uniformly in every direction does a simple factor apply: one candela over a full sphere of 4π steradians gives about 12.566 lumens. For any directional source the relationship depends on the beam distribution.
Not in general, because lumens are weighted by the eye's spectral response. Only for monochromatic light at 555 nanometres is there a fixed relationship, where one watt corresponds to exactly 683 lumens by the definition of the candela.
An older term for the luminous flux of a source expressed as if it radiated uniformly. One spherical candlepower equals 4π lumens, roughly 12.566 lumens, because a full sphere contains 4π steradians.
No. Lumens measure light leaving a source. Lux measures light arriving on a surface, defined as one lumen per square metre, so the same lamp produces different lux readings depending on the area it illuminates.
Since the 2019 SI revision, the candela is defined by fixing the luminous efficacy of radiation at 540 terahertz to be exactly 683 lumens per watt. It is a defined constant rather than a measured one.
Beam distribution, colour temperature and colour rendering all differ independently of total flux. A tightly focused beam appears far brighter in its centre than a diffuse source of identical lumen output.
