Free Engineering Tool · NUREG-1805
View Factor Calculator
The view factor, or configuration factor, is the share of a radiating surface that a target can see. Pick the shape of the source, enter its size and where the target sits, and the calculator returns the factor and the radiant heat flux for the emissive power you enter. Cylinder factors follow NUREG-1805 Fire Dynamics Tools.
- Based on NUREG-1805
- Free, runs in your browser; your inputs stay on your device
- Screening tool, not reviewed or sealed by a licensed engineer
- Last updated October 2026
Screening tool, not engineering advice. This calculator estimates results with simplified published methods. It has not been reviewed or sealed by a licensed engineer for your project, and DGA does not warrant that its results are accurate, complete or suitable for any use. Check every result independently and have the licensed professional responsible for your project review it before you rely on it. Use is subject to the Terms of Use.
Advanced method. The correlations behind this tool come from research and test data with narrow ranges of validity. Outside those ranges results can be wrong by a wide margin, and they are no substitute for a fire model or an analysis by a qualified fire protection engineer.
Geometry
Result
Geometry Sketch
About This Tool
A small target receives radiant heat from a surface in proportion to how much of its hemispherical view the surface fills, weighted toward what lies straight ahead. That proportion is the configuration factor F, from 0 (the source is out of sight) to 1 (the source fills the whole view). With the source treated as a uniform emitter, the incident heat flux is q″ = E F, where E is its emissive power; a black surface at temperature T emits E = σT4, about 107 kW/m² at 900 °C.
Parallel rectangle. For a target facing a rectangle whose corner lies straight in front of it, F = (1/2π) [X/√(1+X²) tan−1(Y/√(1+X²)) + Y/√(1+Y²) tan−1(X/√(1+Y²))], with X and Y the rectangle sides divided by the separation. Factors add, so a target opposite any point is handled by splitting the rectangle into four corner pieces around the target’s foot point. When the target is beyond an edge, the same sum with negative side lengths subtracts the part that is not there.
Perpendicular rectangle. For a target lying in a plane at right angles to the source, such as a point on the ground in front of a burning wall, F = (1/2π) [tan−1(b/c) − c/√(a²+c²) tan−1(b/√(a²+c²))], where a is the rectangle height above the target plane, b its width measured from the target’s line and c the distance. Pieces on either side are added, and a window with its sill above the target plane is the full height minus the strip below the sill.
Cylinder. A pool fire flame is usually modeled as a vertical cylinder with the pool diameter and the flame height. The factors for a vertical target facing the axis and for a horizontal target facing up are the closed-form expressions used with the solid flame model in NUREG-1805; for a target above the base the flame is split into two cylinders at the target’s height and the parts are added. The worst orientation combines the two components as a vector.
Worked example
A window 10 ft wide and 8 ft high (3.05 m by 2.44 m) faces a neighboring wall 10 ft (3.05 m) away. For a point on that wall opposite the center, each quarter of the window is 5 ft by 4 ft, so X = 0.5, Y = 0.4 and each piece gives 0.0501; the four together give F = 0.201. If the flames in the window radiate at 100 kW/m², the wall receives about 20 kW/m². Opposite a corner of the window the factor falls to 0.124 (12 kW/m²). A point on the ground 10 ft out from the base of a burning wall of the same size sees F = 0.055, or 5.5 kW/m².
Assumptions and limits
The source is a flat or cylindrical surface with uniform emissive power, the target is small compared with the distances, and the air between them does not absorb radiation, which is conservative at longer distances. Real flames and openings are not uniform and change with time; smoke can hide much of a flame. The emissive power has to come from a separate estimate such as a flame model or a temperature.
When to bring in an engineer
Configuration factors drive exposure checks between buildings, separation of equipment and storage, and radiation from openings to exterior egress routes. DGA carries these through in fire modeling and code consulting on exposure separation and opening protection.
References
- Iqbal, N., and Salley, M.H., Fire Dynamics Tools (FDTs): Quantitative Fire Hazard Analysis Methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program, NUREG-1805, U.S. Nuclear Regulatory Commission, 2004, Chapter 5: configuration factors for cylindrical flames.
- Howell, J.R., Mengüç, M.P., Daun, K., and Siegel, R., Thermal Radiation Heat Transfer, 7th ed., CRC Press, 2021; and Howell, J.R., A Catalog of Radiation Heat Transfer Configuration Factors, University of Texas at Austin (online).
- Shokri, M., and Beyler, C.L., “Radiation from Large Pool Fires,” Journal of Fire Protection Engineering, 1 (4), 1989, pp. 141 to 150.
- NFPA 921, Guide for Fire and Explosion Investigations, National Fire Protection Association, 2024 edition: radiant heat transfer and configuration factors.
- NFPA 80A, Recommended Practice for Protection of Buildings from Exterior Fire Exposures, National Fire Protection Association, current edition: radiant exposure from openings in an exposing building.
Frequently Asked Questions
What is a view factor in fire protection?
It is the fraction of the radiation leaving a hot surface or flame that reaches a target, set purely by geometry: size, distance, position and orientation. Multiplying it by the emissive power of the source gives the radiant heat flux at the target.
How do you find the view factor when the target is not opposite a corner?
Split the source into rectangles that each have a corner directly in front of the target, compute each corner factor and add them. If the target is beyond an edge, include imaginary rectangles that reach the target and subtract them. The calculator does this automatically from the target position.
Can a view factor be greater than 1?
No. A factor of 1 means the source fills the target’s entire field of view, as for a large flat wall of fire very close to the target. Typical factors at practical separation distances are 0.01 to 0.3.
Which target orientation gives the highest heat flux?
A target turned to face the source squarely receives the most. For a flame cylinder the calculator gives this worst-case value along with the factors for a vertical target facing the flame and a horizontal target facing up, which are the usual cases for walls and roofs or the ground.
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Engineering aid only, provided as is and without warranty. Results depend on the inputs and on the simplifying assumptions of the method summarized on each tool page, and DGA has not verified them for your application. They do not replace code-required design, testing or review by the licensed engineer responsible for the project. Use of the tools is subject to the Terms of Use.