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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

What You'll Need

Godox MS300V MS300-V Compact Studio Strobe Flash Light - 300W,GN58 0.1-1.8S Recycle Time,2.4G X System,Bowens Mount LED Modeling Lamp for Photographic Studio Portrait Shooting(MS300 Upgraded Version)

$119.004.6
View on Amazon
Torjim Softbox Photography Lighting Kit, 16'' x 16'' Professional Softbox Lighting Kit with 85W 3000-7500K LED Bulbs, Studio Lights for Photography/Video Recording/Live Streaming/Portraits Shooting

Torjim Softbox Photography Lighting Kit, 16'' x 16'' Professional Softbox Lighting Kit with 85W 3000-7500K LED Bulbs, Studio Lights for Photography/Video Recording/Live Streaming/Portraits Shooting

$37.974.4
View on Amazon

NEEWER 130cm Parabolic Reflective Umbrella Photography with White Interior, Soft Lighting with Diffuser Cover, Bag, Quick Folding for Camera Studio Video Shooting Monolight Flash,NS2U

$52.634.7
View on Amazon

Godox MS300V MS300-V Compact Studio Strobe Flash Light - 300W,GN58 0.1-1.8S Recycle Time,2.4G X System,Bowens Mount LED Modeling Lamp for Photographic Studio Portrait Shooting(MS300 Upgraded Version)

$119.004.6
View on Amazon
Torjim Softbox Photography Lighting Kit, 16'' x 16'' Professional Softbox Lighting Kit with 85W 3000-7500K LED Bulbs, Studio Lights for Photography/Video Recording/Live Streaming/Portraits Shooting

Torjim Softbox Photography Lighting Kit, 16'' x 16'' Professional Softbox Lighting Kit with 85W 3000-7500K LED Bulbs, Studio Lights for Photography/Video Recording/Live Streaming/Portraits Shooting

$37.974.4
View on Amazon

NEEWER 130cm Parabolic Reflective Umbrella Photography with White Interior, Soft Lighting with Diffuser Cover, Bag, Quick Folding for Camera Studio Video Shooting Monolight Flash,NS2U

$52.634.7
View on Amazon

As an Amazon Associate, we earn from qualifying purchases.

Price as of Jun 27, 2026. Product prices and availability are accurate as of the date indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.

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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: Jul 13, 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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