---
title: "Projector Screen Size & Throw Calculator | Reel Picnic"
description: "Throw ratio and distance to picture size for 16:9, 2.39:1 and 4:3, the distance a screen needs, and a brightness check in nits and foot-lamberts."
url: https://reelpicnic.me/projector-screen-calculator/
---

# Projector screen calculator

How big a picture you'll get from where the projector sits, where to put it for the screen you have, and whether it will be bright enough. Metric and imperial side by side.

## How it works

A projector's **throw ratio** is the distance from the lens to the screen divided by the picture width, so the width is the distance divided by the ratio: 4 m at 1.5 gives a picture 2.67 m wide. A zoom lens covers a range of ratios, and so a range of sizes and of places the projector can go.

The ratio describes the projector's full frame, usually 16:9. A 2.39:1 "scope" film inside it keeps the full width with bars above and below; a 4:3 film keeps the full height with bars at the sides.

Brightness spreads over the whole frame. At gain 1.0, foot-lamberts are the lumens divided by the area in square feet, and candela per square metre (nits) the lumens divided by π times the area in square metres; gain multiplies both. Cinemas aim for 48 cd/m² (14 fL) in a dark room. Treat the answer as a best case: accurate colour modes, eco modes and an ageing lamp all give less light than the spec sheet.

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## Sources

- Throw ratio = distance ÷ picture width, and zoom lenses spanning a range of ratios: [Throw (projector)](https://en.wikipedia.org/wiki/Throw_(projector)), Wikipedia. Makers measure from slightly different points, so check your manual.
- Cinema reference luminance of 48.0 cd/m² (14.0 fL) at the centre of the screen, with a commercial-cinema tolerance of ±10.2 cd/m², and the note that luminance falls as lamps age: [SMPTE ST 431-1:2006, D-Cinema Quality — Screen Luminance Level, Chromaticity and Uniformity](https://pub.smpte.org/doc/st431-1/20060418-pub/st0431-1-2006.pdf).
- A perfectly diffusing surface lit by one foot-candle (one lumen per square foot) has a luminance of one foot-lambert, which is 1/π candela per square foot: [Foot-lambert](https://en.wikipedia.org/wiki/Foot-lambert), Wikipedia. Conversions (1 fL = 3.426 259 cd/m², 1 ft = 0.3048 m, 1 in = 2.54 cm): [NIST Guide to the SI (SP 811), Appendix B.9](https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9).

## Questions

**Where do I find my projector's throw ratio?**

On its spec sheet or in the manual, often written as a range such as "throw ratio 1.40–2.24:1". If the maker only gives a distance for one screen size, divide that distance by the picture width.

**What screen gain should I enter?**

A matte white screen, a painted wall or a white sheet is close to 1.0. Grey screens are usually below 1, and high-gain screens 1.3 or more, which makes the picture brighter straight ahead but dimmer from the sides.

**Is 2000 lumens enough for an outdoor movie?**

It depends on the picture size and how dark it is. Enter your screen size in step 3: if the result is well above the 48 cd/m² cinema reference, it will look good once the sky is properly dark. At dusk the sky adds light, so start later or keep the picture smaller.