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Studio Lighting Calculator — Flash Power & Guide Number for Any Setup

Find exact flash power for any aperture, distance, and modifier

Power Needed

151 Ws

Guide #

25

Status

OK

Estimated Power

151

watt-seconds

Required GN

25

Your Flash f/

f/52

Analysis

Your flash has enough power

At this distance, your flash delivers f/52

Modifier Loss-1.5 stops

Example Calculations

1Portrait at f/8, 6 ft with Softbox

Inputs

Target f-stopf/8
ISO100
Distance6 ft
ModifierSoftbox (small)

Result

Power Needed~200 Ws

GN needed = 8 × 1.83m × 1.68 (softbox loss) = 24.6. Ws ≈ (24.6/2)² ≈ 151. Round up for headroom.

2Group Shot at f/11, 12 ft, Bare Flash

Inputs

Target f-stopf/11
ISO100
Distance12 ft
ModifierNo modifier

Result

Power Needed~508 Ws

GN = 11 × 3.66m = 40.2. Ws ≈ (40.2/2)² ≈ 404. With headroom: ~500 Ws.

Frequently Asked Questions

Q

How do I calculate flash power needed?

Guide Number = f-stop × distance (meters) at ISO 100. Required watt-seconds ≈ (GN/2)². For f/8 at 3m with a softbox (-1.5 stops): adjusted GN = 8×3×1.41 = 33.9, so ~288 Ws needed.

  • GN = aperture × distance at ISO 100
  • Ws ≈ (GN ÷ 2)² (rough estimation)
  • Modifier loss: softbox -1.5 stops, umbrella -1 stop
  • Higher ISO reduces power needed
  • Flash-to-subject distance is the key variable
ModifierLight LossGN FactorBest For
Bare flash0 stops1.0×Hard light, maximum range
Umbrella-0.5 to -1 stop0.7-0.84×General portrait lighting
Small softbox-1.5 stops0.59×Headshots, close range
Large softbox-2 stops0.5×Full body, fashion
Q

What watt-seconds do I need for portraits?

For headshots at 4-6 ft with softbox: 150-300 Ws is plenty. Full body at 8-10 ft: 400-600 Ws. Group shots at 12-15 ft: 600-1000 Ws. Using higher ISO (200-400) reduces power requirements.

  • Headshot (4-6 ft): 150-300 Ws
  • Half body (6-8 ft): 300-400 Ws
  • Full body (8-10 ft): 400-600 Ws
  • Group (12+ ft): 600-1000 Ws
  • Product/tabletop: 100-200 Ws
Q

What is guide number?

Guide Number (GN) rates flash power at ISO 100. GN = f-stop × distance in meters. A flash with GN 58 gives f/8 at 7.25m bare. Higher GN = more powerful flash. Always check if GN is in feet or meters.

  • GN in meters is standard (ISO 100)
  • GN 32-40: speedlights
  • GN 40-60: powerful speedlights
  • GN 60-80: studio monolights (250-500 Ws)
  • GN 80+: high-power studio packs
Q

How does modifier size affect light quality?

Larger modifier relative to subject = softer light. A 4-foot softbox 3 feet from subject is very soft. Same softbox at 10 feet is hard. Rule: modifier should be at least as large as the subject for soft light.

  • Apparent size = physical size / distance
  • Larger apparent size = softer shadows
  • Closer = softer (but less even)
  • Further = harder (but more even)
  • Large modifier + close = best portrait light

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Last Updated: Mar 20, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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