Daylight Factor Calculator
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Calculate
Enter the indoor illuminance in foot-candles.
Enter the outdoor illuminance in foot-candles.
Overview
The Daylight Factor Calculator estimates how much useful daylight reaches an interior space by comparing indoor daylight illuminance with simultaneous outdoor daylight illuminance under a standard overcast-sky assumption. It returns the daylight factor as a percentage and also shows indoor illuminance and outdoor reference illuminance.
This calculator is intended for daylight adequacy screening. It helps you judge whether a room has very weak daylight, limited daylight, practical daylight contribution, or strong daylight availability.
The model is fixed to daylight factor only. It does not perform annual daylight simulation, sun-path analysis, glare probability analysis, solar heat gain analysis, or climate-based daylight autonomy. IES treats daylight factor as a valid daylight metric, but also distinguishes it from broader annual daylight performance methods.
How to Use This Calculator
Enter indoor illuminance — the measured or estimated illuminance at the point being evaluated, in lux or foot-candles.
Enter outdoor illuminance — the simultaneous outdoor illuminance reference, in lux or foot-candles.
Click "Calculate" — review the daylight factor, indoor illuminance, outdoor illuminance, and daylight adequacy interpretation.
Use the result to judge whether the space is likely to rely heavily on electric lighting or receive practical daylight during daytime.
Inputs & Outputs
Inputs
- •Indoor Illuminance (lux / fc)
- •Outdoor Illuminance (lux / fc)
Outputs
- •Daylight Factor (%)
- •Indoor Illuminance (lux / fc)
- •Outdoor Illuminance (lux / fc)
Formula
Calculator Formula
Daylight Factor (%) = (Indoor Illuminance / Outdoor Illuminance) × 100
This formula applies identically in both unit systems. Because both numerator and denominator use the same illuminance unit (lux or foot-candles), the result is always a dimensionless percentage.
Engineering Reference
The daylight factor is the ratio of indoor illuminance at a point inside a room to the simultaneous outdoor illuminance under a standard CIE overcast sky, expressed as a percentage. It is one of the standard daylight performance metrics recognized by IES.
Calculator Variables
| Variable | Meaning | Units |
|---|---|---|
| Indoor Illuminance | Illuminance at the evaluation point inside the room | lux / fc |
| Outdoor Illuminance | Simultaneous outdoor illuminance reference | lux / fc |
| Daylight Factor | Ratio of indoor to outdoor illuminance | % |
What is Daylight Factor
Daylight Factor is the ratio of indoor illuminance at a point inside a room to the simultaneous outdoor illuminance under a standard overcast sky, expressed as a percentage. In simple terms, it tells you how much of the available outdoor daylight reaches the interior point being studied.
A higher daylight factor usually means stronger daylight penetration and lower daytime dependence on electric lighting, while a lower daylight factor usually means the opposite. IES identifies daylight factor as one of the standard daylight performance metrics, and EN 17037 is one of the key current daylight standards in Europe.
Why Daylight Factor Matters
Daylight factor screening helps designers and engineers quickly assess whether a room receives enough natural light for its intended use. Spaces with very low daylight factors are likely to depend heavily on electric lighting throughout the day, increasing energy consumption. Spaces with high daylight factors may benefit from reduced lighting energy but may also need glare control and shading strategies.
Accurate daylight assessment supports energy-efficient building design, occupant comfort, and compliance with daylight standards such as EN 17037.
Engineering Applications
Daylight factor calculations are used across architectural and building engineering disciplines. Architects use them during early design to evaluate window sizing and room depth. Lighting designers use daylight factor to coordinate electric lighting with available daylight. Building energy consultants use daylight screening to support energy modeling and code compliance.
In all cases, daylight factor provides a quick, standardized way to compare daylight performance across different room configurations, glazing options, and building orientations.
Illuminance Unit Conversions
The following table provides common unit conversions used in daylight calculations:
| Unit | Equivalent |
|---|---|
| 1 lux | 0.0929 fc |
| 1 fc | 10.764 lux |
| 1,000 lux | 92.9 fc |
| 10,000 lux | 929 fc |
Practical Tips
When estimating daylight factor, always ensure that indoor and outdoor illuminance values are measured or estimated under the same sky condition and at the same time.
For indoor measurements, place the sensor at the work plane height (typically desk level, about 0.8 m or 30 inches above the floor) at the point of interest.
For outdoor measurements, use an unobstructed horizontal illuminance reading that represents the available sky luminance without direct sun interference.
Important: This calculator is a quick daylight screening tool. Final daylight design should be verified using detailed simulation tools that account for room geometry, surface reflectances, glazing properties, and annual sky conditions per IES and EN 17037 guidance.
Key Facts
- Daylight Factor is a point-in-time daylight metric, not an annual performance metric. IES distinguishes daylight factor from annual daylight simulation methods.
- EN 17037 is the modern European standard often referenced in daylight design discussions and has shifted attention toward practical daylight availability and distribution.
- A room can have a positive daylight factor but still need electric lighting for demanding visual tasks, because daylight factor indicates daylight ratio, not task suitability by itself.
- High daylight factor values can improve daytime daylight availability, but large glazing can also increase the need to review glare and solar-control strategy.
- The standard CIE overcast sky is the reference condition for daylight factor calculations, providing a uniform diffuse illumination model.
Applications
- Early-stage room daylight screening.
- Comparing glazing options.
- Evaluating daylight penetration at a desk or work point.
- Quick daylight checks for classrooms, offices, residential rooms, and corridors.
- Preliminary natural-light assessment before detailed simulation.
- Supporting electric-lighting control decisions in daylit zones.
Example Calculation
Metric Example
Given:
- Indoor Illuminance = 180 lux
- Outdoor Illuminance = 9,000 lux
Calculation:
Daylight Factor = (180 / 9,000) × 100
Daylight Factor = 2.0%
Result: Daylight Factor = 2.0%
Interpretation: A daylight factor of 2.0% is at the lower edge of the adequate daylight range. The space is likely to receive a practical daylight contribution during daytime, but electric lighting may still be needed depending on the task and room depth.
Imperial Example
Given:
- Indoor Illuminance = 18 fc
- Outdoor Illuminance = 900 fc
Calculation:
Daylight Factor = (18 / 900) × 100
Daylight Factor = 2.0%
Result: Daylight Factor = 2.0%
Interpretation: This imperial case produces the same 2.0% daylight factor, which indicates a practical but not exceptionally strong daylight condition.
Standards & References
- ANSI/IES LP-3 — Designing and Specifying Daylighting for Buildings — recognizes daylight factor as a standard daylight performance metric
- EN 17037 — widely referenced European daylight standard used in modern daylight design discussions
- IES Lighting Library — current IES standards collection and lighting guidance framework
Limitations
- This calculator is a screening tool, not a full daylight simulation tool.
- It assumes a standard overcast-sky daylight-factor concept.
- It does not evaluate annual daylight autonomy, direct sunlight, glare probability, solar heat gain, shading schedules, or occupancy timing.
- It does not prove that the daylight level is suitable for a specific task unless a task requirement is evaluated separately.
- It does not replace detailed daylight simulation or lighting design guidance from IES or project-specific standards.
Common Mistakes to Avoid
- Using indoor and outdoor illuminance values taken under different conditions.
- Comparing lux inputs with foot-candle inputs without conversion.
- Assuming daylight factor alone proves visual comfort.
- Treating daylight factor as an annual daylight metric.
- Ignoring room depth and interior reflectance.
- Ignoring glare risk when daylight factor is high.
- Using unrealistic outdoor illuminance values.
- Assuming a good daylight factor always eliminates the need for electric lighting.
Frequently Asked Questions
What does this calculator measure?
Is daylight factor the same as annual daylight simulation?
Is a higher daylight factor always better?
What does a daylight factor below 1% mean?
What does a daylight factor between 2% and 5% mean?
Can I use lux or foot-candles?
Does this calculator tell me if glare will be a problem?
Is this enough for final daylight design?
Frequently Used Together
Engineers often use these calculators in combination for complete project workflows:
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Calculate
Enter the indoor illuminance in foot-candles.
Enter the outdoor illuminance in foot-candles.