Calculator inputs
Example data table
| Deck size | Layout | Target (fc) | Light output | Typical spacing | Typical count |
|---|---|---|---|---|---|
| 12 × 10 ft | Grid | 2.0 | 60 lm | 4–6 ft | 6–10 |
| 16 × 12 ft | Both | 2.0 | 80 lm | 5–7 ft | 10–16 |
| 20 × 14 ft | Grid | 3.0 | 100 lm | 4–6 ft | 14–22 |
| 24 × 16 ft | Perimeter | 1.5 | 60 lm | 6–8 ft | 12–18 |
| 30 × 18 ft | Both | 2.5 | 120 lm | 5–7 ft | 22–34 |
Formula used
- 1 Deck area (ft²) = length × width.
- 2 Lumens required = area × target_fc ÷ (UF × MF).
- 3 Light count = (lumens_required ÷ lumens_per_light) ÷ uniformity.
- 4 Grid spacing ≈ √(usable_area ÷ grid_lights), then clamped.
- 1 Usable perimeter uses an inside offset rectangle.
- 2 Perimeter lights ≈ usable_perimeter ÷ perimeter_spacing_target.
- 3 Both layout splits edge definition and area coverage.
- 4 Transformer size adds 20% headroom for reliability.
This calculator gives planning estimates. For final layouts, confirm fixture photometrics and local electrical requirements.
How to use this calculator
- Choose units, then enter deck length and width.
- Set edge offset to keep lights off rails.
- Pick a layout: grid, perimeter, or both.
- Enter your target brightness in foot-candles.
- Enter lumens and watts for your chosen fixtures.
- Use factors for shading, dirt, and uniformity.
- Add step or post lights if needed.
- Press Calculate to see spacing above the form.
- Download CSV or PDF to share with installers.
Lighting goals for garden decks
Low-level deck lighting typically targets 1–4 foot-candles (fc) at the walking surface. For relaxed entertaining, many planners start near 2.0 fc, then adjust for darker materials, nearby planting, and rail shadows. This calculator converts your deck area into a lumen requirement so your plan scales with size.
Choosing fixture output
Deck step and accent lights often produce 30–120 lumens each. Smaller outputs create softer pools, but you’ll need more fixtures for the same fc target. If you know beam shape or shielding, lower the utilization factor (UF) to 0.50–0.65 to reflect real delivered light. Open layouts may support UF around 0.70–0.80.
Spacing logic and layout
A grid layout aims for even coverage across the usable area (after edge offset). The rule-of-thumb spacing uses √(usable_area ÷ grid_lights) and then clamps it between your minimum and maximum spacing. For perimeter-only layouts, the calculator estimates count from usable perimeter ÷ target perimeter spacing, then clamps the resulting spacing to avoid dark gaps.
Maintenance factor (MF) accounts for lens dirt, aging LEDs, and small voltage losses; values near 0.80 are common for outdoor fixtures. Uniformity factor (0.60–0.85) increases counts to reduce scalloping and shadow bands. If your deck borders a lawn path, consider 'both' layout so edges read clearly without over-lighting the center and keeps corners from feeling dim.
Power planning and reserves
Once the fixture count is set, total wattage is lights × watts per light. A common reliability practice is 20% headroom on the power supply, which this calculator applies to suggest a transformer size. Adding 10–30% reserve lights is helpful when matching batches, planning replacements, or expanding along stairs and posts.
Field checks before installation
Use the output spacing to mark reference points, then verify sightlines at night. Check that step lights avoid glare at seating height and that post lights don’t create harsh hotspots. If long cable runs are unavoidable, plan shorter branches or heavier gauge wire to reduce voltage drop and preserve lumen output.
FAQs
1) What target brightness should I use for a deck?
Many decks feel comfortable around 1.5–2.5 fc for walking and seating. Choose closer to 3–4 fc for task areas like grills. If glare is a concern, lower the target and add more low-lumen fixtures.
2) Why does the calculator ask for utilization and maintenance factors?
These factors estimate real light reaching the surface. UF reflects shielding, beam spread, and obstructions. MF accounts for dirt, aging, and small voltage losses. Lower values increase the recommended fixture count for a safer plan.
3) How do I pick a good spacing range?
Set a minimum that avoids harsh hotspots, then a maximum that avoids dark gaps. Many low-level deck fixtures land between 4 and 8 feet. Use the clamp values to keep spacing practical even on unusual deck sizes.
4) Should I choose grid, perimeter, or both?
Grid supports even coverage across the deck. Perimeter emphasizes edges and pathways. Both works well when you want a defined border plus a comfortable center zone. If you have stairs, add step lights regardless of layout.
5) Why is transformer size higher than total wattage?
Outdoor systems perform better with headroom. The calculator adds 20% to reduce overheating and allow future expansion. If cable runs are long, extra margin helps offset voltage drop and keeps brightness more consistent.
6) Can I use meters and still get accurate results?
Yes. Inputs and outputs follow your selected units. Internally, calculations convert to feet for the brightness math, then convert back for display. Double-check fixture specs, since many products list lumens and wattage in the same units worldwide.