Video & LED calculator
Projector Brightness Calculator (Lumens to Foot-Lamberts)
Screen brightness in foot-lamberts equals projector lumens times screen gain divided by screen area in square feet: 10,000 lumens on a 16 by 9 foot matte screen yields about 69 fL. Enter your projector and screen to see whether the image survives the room.
Matte white = 1.0. High-gain surfaces trade brightness for viewing angle and hot-spotting.
Formulas
Foot-lamberts
fL = lumens × gain / area(sq ft)- gain:
- screen gain relative to matte white (1.0)
- area:
- screen width × height in square feet
Nits from foot-lamberts
nits = fL × 3.426How it works
Projector lumens spread across the screen, so the same projector gets dimmer as the screen grows: doubling screen width quarters the brightness. Foot-lamberts measure what actually reaches the eye, which is why screen size belongs in every lumens conversation.
The target depends entirely on ambient light. Dark cinema conditions are standardized around 14 to 16 fL (SMPTE); corporate ballrooms with the lights politely dimmed want several times that, and a screen fighting daylight loses regardless of lumens, which is when LED walls take the job. Contrast against ambient, not raw brightness, is what the eye reads, and modern AV standards (AVIXA’s viewing standards) specify required contrast ratios by task for exactly that reason.
Screen gain redistributes light rather than creating it: a 1.3-gain surface brightens the on-axis view at the cost of side seats and hot-spotting. Blended multi-projector arrays add another wrinkle, since overlap zones must be blended down to the single-projector level; the edge blending entry covers it.
Worked example: Ballroom keynote: 12,000 lumens on a 20 × 11.25 ft screen, matte white
- 1.Area: 20 × 11.25 = 225 sq ft.
- 2.Brightness: 12,000 × 1.0 / 225 = 53 fL.
- 3.With stage wash spilling 5 to 10 fL of ambient onto the screen, the image holds roughly a 6:1 contrast: workable for slides, marginal for dark video.
About 53 fL: solid for a dim ballroom, tight if the lighting designer wins the argument.
Lumens needed by screen size and room
| Screen (16:9) | Area | Dark room (16 fL) | Dim ballroom (45 fL) |
|---|---|---|---|
| 12 × 6.75 ft | 81 sq ft | 1,300 lm | 3,600 lm |
| 16 × 9 ft | 144 sq ft | 2,300 lm | 6,500 lm |
| 20 × 11.25 ft | 225 sq ft | 3,600 lm | 10,100 lm |
| 24 × 13.5 ft | 324 sq ft | 5,200 lm | 14,600 lm |
| 32 × 18 ft | 576 sq ft | 9,200 lm | 25,900 lm |
Field notes
- Ambient light on the screen is the real enemy: shielding the screen from wash and moving the warmup look off the projection surface buys more contrast than another 2,000 lumens.
- For blends, compute per projector against its own coverage area, not the total canvas.
Frequently asked questions
How many lumens do I need for a projector?
Work backward from foot-lamberts: multiply your screen area (sq ft) by the target (16 fL dark room, 40 to 60 fL for a typical dim ballroom) and divide by screen gain. A 200 sq ft screen at 50 fL needs about 10,000 lumens.
What is a good foot-lambert level for presentations?
Cinema standardizes 14 to 16 fL in the dark. Corporate practice aims far higher because rooms are never truly dark: 30 to 50 fL is a common working range, more if the screen shares the stage with wash light.
Are projector lumens and ANSI lumens the same?
Use ANSI (or ISO) lumen ratings for this math; they measure averaged white output under a defined method. Marketing "LED lumens" and peak figures on consumer projectors overstate real output, sometimes by several times.