Start with target illuminance, then compare fixture counts
A higher wattage does not by itself tell you how much light will reach a desk or how evenly a room will be illuminated.
Watts describe electrical input, lumens describe light output, and lux describes illuminance on a surface.
This calculator helps homeowners and small shop owners compare two luminaire proposals before ordering.
It uses the same room area and target maintained average illuminance to calculate an integer fixture count, estimated average illuminance, connected power and product budget.
A and B are alternative installations, each lighting the space on its own.
Do not add their counts together as a mixed lighting design.
The model uses product light output rather than a generic renovation price per square metre.
Treat the result as a set of conditions to review with a lighting supplier using photometric data and the proposed layout.
Input meanings: m², lx, lm and W
Area and target
Area A is the area of the calculation workplane in square metres.
Use the actual room or task area, consistently, rather than a property listing’s total area.
Target E is the maintained average illuminance in lux.
Record the task, workplane height and reference used to select it so the supplier can reproduce the assumptions.
Light output and power
F is the output leaving one complete luminaire, measured in lumens.
Bare-lamp or LED-chip lumens may differ from luminaire output.
Enter electrical input W including the driver.
Two products with equal wattage can need different quantities because of output, distribution and room conditions.
Price and quoted quantity
Enter the price of one complete fixture on a consistent tax and delivery basis.
A blank price means the product budget is unknown; an explicit zero means free.
The optional quoted count lets you test an existing proposal against the same illuminance assumptions.
Leaving it blank does not stop the required-count calculation.
Currency and evidence
KRW and USD change the currency label only; no exchange-rate conversion occurs.
Use one currency for both candidates.
Record the model, specification revision and quote date.
These details preserve the comparison when a supplier later changes the specification or offer.
UF and MF belong to the product and room
The utilisation factor UF expresses the share of the entered luminaire output effectively reaching the calculation workplane.
Room geometry, mounting height, surface reflectances and light distribution affect the appropriate value.
This model uses a factor referenced to luminaire-output lumens.
Do not copy a CU or UF referenced to bare-lamp lumens without reconciling the definition, or apply optical losses again when they are already included in the output figure.
The maintenance factor MF accounts for light reduction over time, including source ageing and dirt on luminaires and room surfaces.
Its selection depends on the product, environment and cleaning or replacement plan.
A long advertised lifetime alone does not establish the factor.
If candidates have different photometry or maintenance conditions, enter separate factors for each.
Single values or unconfirmed ranges
Single-value mode requires a target reference, product reference and factor reference.
Providing text is an evidence-recording step, not independent verification or certification.
If data is unavailable, choose range mode and supply your own lower and upper factors.
The calculator provides no recommended default range and always labels these results as scenarios.
The supported factor interval of 0.001–1 is an input limit, not a design recommendation.
Lumen-method formulas and rounding
N = ceil(E × A / (F × UF × MF))
Emaint = N × F × UF × MF / A
Connected power = N × W
Product budget = N × unit price
Divide target illuminance times area by the effective lumens from one luminaire.
Round the resulting quantity upward because fixtures are purchased as whole units.
Rounding down or to the nearest integer can leave the estimated average below the target.
The integer count is then substituted back into the illuminance formula to calculate the maintained average and target margin.
Display rounding does not replace the underlying calculation precision.
If you enter quoted count Q, the quoted illuminance uses Q in place of N.
A negative quote margin means the estimated average is below the target under that row’s assumptions.
Additional count is max(0, N−Q).
It does not establish where those additional fixtures should be mounted or whether the electrical installation can accommodate them.
How to use the comparison
- Define the room or task area and obtain a target maintained average illuminance.
Document the applicable task, workplane and target reference.
- Read complete-luminaire output and driver-inclusive input power from each specification.
Check that values belong to the same model, colour temperature and optical option.
- Select single-value mode for documented factors or range mode for unconfirmed assumptions.
Enter decimal fractions, such as 0.6 for 60%, with lower values no greater than upper values.
- Enter comparable unit quotes and optional proposed counts.
Compare integer counts, average illuminance, power and budget, while retaining any missing-data status for the other candidate.
- Save the review as TXT or print the input conditions and result table.
Ask the lighting supplier to check the photometry, layout and post-installation measurement plan before you order.
Worked example: 20 m² and a hypothetical 300 lx target
These figures illustrate the arithmetic, not recommended lighting levels or current market prices.
Assume 20 m², 300 lx, UF 0.6 and MF 0.8 for both candidates.
Candidate A provides 1,000 lm at 10 W for KRW 20,000 per fixture; candidate B provides 1,500 lm at 14 W for KRW 28,000.
Hypothetical candidate counts, maintained average illuminance, power and product budgets| Candidate | Count | Average | Power | Budget (KRW) |
|---|
| A | 13 | 312 lx | 130 W | 260,000 |
| B | 9 | 324 lx | 126 W | 252,000 |
Why 12.5 becomes 13
For A, 300×20÷(1,000×0.6×0.8)=12.5.
Thirteen fixtures produce an estimated 312 lx, a margin of 12 lx.
A quote for 12 fixtures gives 288 lx, a −12 lx margin and one additional fixture under this model.
For B, a quote for eight fixtures also gives 288 lx; nine fixtures give 324 lx.
The lower count and budget do not establish superior glare control, distribution or installation suitability.
Reading the UF / MF range scenarios
For the same 20 m², 300 lx and 1,000 lm, suppose you enter UF 0.5–0.7 and MF 0.7–0.9.
The four lower/upper combinations require 18, 14, 13 and 10 fixtures respectively.
The lowest two factors require 18; the highest two require 10.
The calculator does not decide whether these ranges represent your room.
Each row changes the count too
The 18-fixture low-factor row and 10-fixture high-factor row both produce 315 lx.
This does not mean ten fixtures produce 315 lx throughout the factor range.
Each row recalculates the count needed for its own factor pair.
To inspect one fixed installation count, enter that count as the quoted quantity and compare the quoted lux column across all rows.
Equal lower and upper values produce only one unique combination.
Turn the results into a purchase review
Review lighting assumptions first
Check that each source applies to the intended installation before relying on the count range.
An unexpectedly large count can indicate incorrect area units, product output or percentage-style factor entry.
The calculator withholds counts above 1,000,000 so an extreme input is not presented as an order quantity.
Separate products from installation
The product budget is count times unit price.
Wiring, ceiling work, removal, controls, labour, delivery and tax are not added automatically.
Connected watts are not an electricity bill, which also needs usage hours, tariff and standby assumptions.
Neither the cheapest product total nor the lowest power automatically identifies the best lighting solution.
Practical home and small-shop scenarios
- For a living-room replacement, enter the existing proposed fixture count to see its target margin.
A reading desk may still require local task lighting even when the room average is sufficient.
- For a small shop, discuss how displays and shelving affect light distribution and reflections.
General circulation lighting and accent lighting should be considered separately.
- For a rented property renovation, compare the required quantity with existing cut-outs and wiring locations.
Additional fixtures can imply extra ceiling and electrical work outside the product budget.
- When comparing two suppliers, use the same area, target and price-inclusion basis.
If their factor evidence differs, resolve that difference before interpreting the quantity or cost gap.
Limits of average-illuminance estimates
The model estimates maintained average illuminance in a reasonably uniform space.
It does not calculate point illuminance, dark corners, shadows, uniformity, glare, colour temperature, colour rendering or flicker.
It does not certify Korean KS, workplace, emergency-lighting or other jurisdictional compliance, or approve electrical designs.
Daylight and existing supplementary lighting are not added automatically.
Check actual illuminance at the specified workplane after installation.
Increasing quantity alone may not resolve a poor layout.
Light concentrated near walls or blocked by furniture can produce very different experiences at the same room average.
Review the actual photometric file and room plan, and maintain the cleaning and replacement assumptions used for MF.
Initial brightness and maintained average illuminance are different quantities.
No country-specific target table is built into this 2026 implementation.
Frequently asked questions
Is 300 lx a recommended living-room target?
No.
It is a hypothetical worked-example input.
Obtain a target appropriate to the task and workplane from the applicable reference or lighting designer.
Can I reuse UF 0.6 and MF 0.8?
They are example values, not defaults.
Check the product distribution, room geometry, reflectances and maintenance plan.
If evidence is unavailable, enter your own range and interpret it only as scenarios.
Can wattage alone determine the count?
No.
There is no fixed conversion from electrical watts to lumens.
Obtain the complete-luminaire output from the specification; this calculator does not invent a product efficacy.
Why round upward rather than to the nearest integer?
A fractional requirement is rounded upward to avoid an average below the target.
That arithmetic does not guarantee that every point in the room meets the target.
What do a blank price and zero price mean?
Blank means budget unknown, while zero means an explicitly free product.
Counts, illuminance and power can still be calculated without price.
Changing the currency does not convert the numbers.
Can I order the smallest number in range mode?
That count uses the highest entered factors.
To evaluate the same fixed count under lower factors, enter it as the quoted quantity and inspect every row before obtaining a design review with actual evidence.
Does entering a reference certify the single-value result?
No.
It records the basis and distinguishes missing evidence; it does not authenticate the source or establish applicability.
Layout, uniformity, glare and compliance require separate review.
What does the saved review contain?
The TXT includes area, target, currency, product inputs, factors, references, example provenance, candidate status and calculation rows.
Editing a hypothetical example preserves its example-origin label.
Save it with the supplier quotation.
Sources, review date and your next step
Method references were checked on 2026-10-01.
They are not a current product catalogue or a national illuminance schedule.
Recheck the target and UF / MF when product photometry, room conditions or maintenance plans change.
Take the conditions to your lighting supplier
Enter your target and candidate specifications, then save the review.
Request luminaire output data, photometric files, room-specific factors and an installation measurement plan.
For a separate Korean renovation budget, see the interior cost calculator.
Enter target lux and candidate fixtures