Input room details
Results
Room area:
Base target illuminance: lux
Design target after margin: lux
Total lumens required: lm
Recommended fixtures:
Achieved illuminance (recommended): lux
Achieved illuminance (existing): lux
Room cavity ratio (approx):
Total load (recommended):
Lighting power density:
Estimated monthly energy:
Estimated monthly cost:
Summary table
| Parameter | Value | Unit |
|---|
Example illuminance levels
These examples use typical target values and medium-sized rooms.
| Room type | Example size | Area (m²) | Target illuminance (lux) | Total lumens required |
|---|---|---|---|---|
| Bedroom | 4 m × 3 m | 12 | 100 | 1,200 lm |
| Living room | 5 m × 4 m | 20 | 150 | 3,000 lm |
| Kitchen | 4 m × 3.5 m | 14 | 300 | 4,200 lm |
| Home office | 3 m × 3 m | 9 | 300 | 2,700 lm |
Formula used
The calculator is based on the lumen method, commonly used in lighting design for interior spaces.
- Area (A) is the floor area of the room in square metres.
- Target illuminance (E) is the desired lighting level in lux at the working plane.
- Lumens per fixture (F) is the light output of one luminaire or lamp in lumens.
- Coefficient of utilization (CU) accounts for how effectively the fixture delivers light to the working plane.
- Light loss factor (LLF) accounts for lamp depreciation, dirt, and other real-world losses over time.
- Design margin increases the base target to give a more robust layout.
The required number of fixtures (N) is estimated using:
N = (Edesign × A) / (F × CU × LLF)
where Edesign is the base target lux multiplied by one plus the design margin.
The calculator rounds this value up to the next whole fixture to ensure the design target illuminance is achieved or slightly exceeded.
The achieved illuminance is calculated from the chosen number of fixtures by rearranging the same relationship:
Eachieved = (F × CU × LLF × N) / A
The room cavity ratio (RCR) is approximated to help understand how ceiling height and room proportions influence utilization:
RCR ≈ 5 × Hc × (L + W) / (L × W), where Hc is the mounting height above the work plane.
How to use this calculator
- Choose your preferred length units and enter the room length and width using the internal finished dimensions.
- Select the room type. The calculator will suggest a typical base target illuminance value for that application.
- Adjust the base target illuminance and design margin if you want brighter or softer lighting than the suggested level.
- Choose whether you want to enter lumens per fixture directly or use fixture power and luminous efficacy to calculate lumens.
- Enter CU and LLF or keep the defaults if you do not have detailed manufacturer data; select an appropriate surface reflectance description.
- Provide ceiling height and work plane height to estimate room cavity ratio and confirm that your CU assumptions are reasonable.
- Optionally, enter how many fixtures you already have installed and typical operating hours, days, and energy rate for energy and cost estimates.
- Click “Calculate lighting” to see the required fixture count, illuminance levels, power density, and monthly energy and cost estimates, then export results as CSV or PDF.
Recommended lux levels for common rooms
Use lower lux values for relaxing rooms and higher levels for tasks. For more focused bedroom planning, compare these numbers with the dedicated bedroom lighting calculator to fine tune bedside and wardrobe lighting layouts.
Matching fixture output to room size
Large rooms often need several fixtures to avoid bright spots and dark corners. This tool combines area, CU, and LLF so you can choose suitable lumen packages and fixture counts instead of relying on rough rules of thumb.
Comparing lighting with other home energy uses
Lighting is only one contributor to your electricity bill. You can compare the monthly cost from this tool with results from an EV charging cost at home calculator to see how lighting sits alongside transport energy usage.
Fine-tuning bedroom lighting for comfort
Bedrooms usually benefit from dimmer ambient lighting with brighter local task areas. Combine this general tool with the more focused bedroom lighting calculator when you are balancing reading comfort, wardrobe visibility, and relaxing evening scenes.
Integrating lighting with ventilation and comfort
In bathrooms and kitchens, lighting competes for ceiling space with fans and ducts. Use this calculator alongside an exhaust fan size calculator to coordinate airflow requirements and lighting levels for a comfortable, safe environment.
Documenting lighting plans for renovation projects
The CSV and PDF exports help you share lighting assumptions with electricians and designers. Save separate files for each room so you can track fixture counts, power density, and target lux values as your renovation scope evolves.
Frequently asked questions
What lux level should I use for a bedroom?
Many designers choose around 100 lux for general bedroom lighting, with higher task lighting at dressing tables or reading areas. Use this tool and the bedroom lighting calculator to refine choices.
Why do I need coefficient of utilization and light loss factor?
They bridge the gap between theoretical lamp lumens and usable light on the work plane. CU captures geometry and reflectance. LLF allows for aging and dirt so your design stays robust.
Can I mix fixtures with different lumen outputs?
Yes. For quick checks, use an average lumen value per fitting. For detailed design, calculate lumens separately for each group of fixtures and ensure the combined output still meets your target.
How accurate are the energy and cost estimates?
They are planning-level estimates based on entered power, hours per day, days per month, and tariff. Actual bills depend on usage patterns, other appliances, taxes, and supply charges on your account.
What happens if my existing fixtures already exceed the target lux?
The tool will show achieved illuminance above the design target. You might choose lower wattage lamps, dimmer controls, or fewer active fittings to improve comfort and reduce energy use.
Should I design to the base target or design target lux?
Design target lux includes your safety margin for aging and dirt. It is usually safer to size fixtures to meet the design target while allowing dimming or separate switching for flexibility.
Can I use this calculator for commercial spaces?
You can use it for preliminary checks, but commercial projects often follow stricter standards and require professional design. Treat results here as screening tools, not replacements for detailed engineering calculations.