If you’re replacing or upgrading a flood light bulb, knowing the exact flood light bulb dimensions is essential. The wrong size can lead to poor fit, reduced performance, or even damage to your fixture. Most flood lights use standardized shape codes like PAR and BR, where numbers represent eighths of an inch in diameter. Understanding this system helps you choose the correct bulb without guesswork.
This guide breaks down the most common flood light bulb dimensions, explains how to read the coding system, and shows how physical size affects installation and performance. You’ll learn the differences between PAR and BR bulbs, when to use MR or corn bulbs, and how to match a bulb’s dimensions to your fixture’s constraints. Whether for outdoor security, recessed ceiling lighting, or industrial use, getting the size right ensures optimal brightness, beam control, and longevity.
Decode the Bulb Size Code
Understanding the standardized naming system is the first step in identifying flood light bulb dimensions.
What the Letters Mean
The letter in a bulb code indicates its shape and function:
– PAR: Parabolic Aluminized Reflector, rugged, directional, ideal for outdoor flood lighting.
– BR: Bulged Reflector, softer, wider beam, commonly used in indoor recessed fixtures.
– MR: Multifaceted Reflector, small, low-voltage directional bulbs (e.g., MR16).
– A: Standard household shape (e.g., A19), sometimes used in open flood fixtures.
These letters tell you not just the shape but also the light distribution and typical use case.
How Numbers Reveal Diameter
The number in the code represents the bulb’s diameter in eighths of an inch:
– Formula: Diameter (inches) = Number ÷ 8
– Example: PAR38 = 38 ÷ 8 = 4.75 inches wide
This rule applies to PAR, BR, and A-series bulbs. MR-series is an exception because its number is in millimeters (e.g., MR16 = 16mm diameter).
| Code | Diameter (Inches) | Diameter (mm) |
|---|---|---|
| PAR16 | 2.0″ | ~51mm |
| PAR20 | 2.5″ | ~64mm |
| PAR30 | 3.75″ | ~95mm |
| PAR38 | 4.75″ | ~121mm |
| BR30 | 3.75″ | ~95mm |
| BR40 | 5.0″ | ~127mm |
Always measure your fixture opening to confirm compatibility.
Why Length Matters Too
While the code defines diameter, length varies by type:
– PAR bulbs are shorter, designed to sit flush in recessed or weatherproof housings.
– BR bulbs have longer bodies and often protrude slightly from fixtures.
Even with the same diameter (e.g., PAR30 vs BR30), the length difference can determine whether a bulb fits in a shallow can or causes glare.
PAR Bulb Dimensions and Uses
PAR bulbs are the go-to for outdoor and high-performance flood lighting.
PAR38: Standard Outdoor Flood
The PAR38 is the most common outdoor flood light bulb:
– Diameter: 4.75″ (121mm)
– Short body for flush mounting
– Rugged glass resists weather and impact
– Ideal for: Security lights, porch fixtures, landscape uplighting
Because it sits flush, it minimizes glare and fits most weatherproof housings. LED versions offer 1000–2000+ lumens, perfect for illuminating driveways or yards.
PAR30: General Purpose Flood
Slightly smaller than PAR38:
– Diameter: 3.75″ (95mm)
– Used in both indoor and outdoor recessed fixtures
– Common in retail displays, track lighting, and soffit lighting
Available in flood (wide beam) and spot (narrow beam) versions with the same size but different optics.
PAR20 and PAR16: Compact Options
- PAR20 (2.5″): Fits tight recessed or track fixtures and is used in accent lighting.
- PAR16 (2.0″): Smallest in the PAR family, often found in display cases or under-cabinet lighting.
Both are less common for large-area flooding but useful where space is limited.
BR Bulb Dimensions and Applications
BR bulbs deliver softer, more diffused light than PAR, making them ideal for residential interiors.
BR30: Residential Recessed Flood
The BR30 is a staple in home ceiling lighting:
– Diameter: 3.75″, same as PAR30
– Longer body than PAR30, so it may protrude
– Frosted lens diffuses light evenly across walls and ceilings
Best for living rooms, hallways, and bedrooms where soft shadows and ambient glow are preferred.
Pro Tip: If your recessed can is shallow, measure depth before installing a BR30 because it may not fit flush.
BR40: High-Output Indoor Flood
Larger and brighter:
– Diameter: 5.0″ (127mm)
– Used in high-ceiling commercial spaces or large rooms
– Delivers wide, even light distribution
Common in warehouses, garages, or open-plan homes needing broad illumination.
BR vs PAR: Which Fits Your Fixture?
| Factor | Choose BR | Choose PAR |
|---|---|---|
| Fixture Depth | Deep recessed cans | Shallow or flush-mount |
| Light Quality | Soft, diffused | Focused, punchy |
| Location | Indoor | Outdoor/Exposed |
| Glare Control | May protrude and cause glare | Flush fit reduces glare |
Always check your fixture’s max bulb size and depth before choosing.
Niche Flood Light Bulb Sizes
Beyond PAR and BR, other bulb types serve specialized flood lighting roles.
MR16 and MR11: Directional Mini Floods
- MR16: 16mm (0.63″) diameter, pin base (GU5.3)
- MR11: 11mm (0.43″) diameter, smaller pin base
Used in:
– Track lighting
– Landscape spotlights
– Display lighting
– Smart RGB flood fixtures
Despite small size, LED versions can output 500+ lumens with tight beam control.
Corn Bulbs: Industrial-Grade Floods
No standard code, named for their “corn cob” LED array:
– Replace 250W–1000W HID/Metal Halide fixtures
– Output: 5,000–20,000+ lumens
– Used in: Warehouses, gymnasiums, parking garages
Dimensions vary by model, but typically 6–12 inches long and 3–5 inches wide. Requires a compatible high-bay fixture.
A-Shape Bulbs in Flood Fixtures
Standard household bulbs like A19 (2.375″) or A25 (3.125″) are sometimes used in open flood fixtures:
– Omnidirectional light, so fixture housing controls beam spread
– Less efficient than reflector bulbs
– Common in decorative or retrofit outdoor fixtures
Only suitable if the fixture provides beam shaping and weather protection.
How to Measure for Fit
Getting the right flood light bulb dimensions starts with measuring your fixture.
Step 1: Check Fixture Opening
Use a tape measure or caliper:
– Measure the diameter of the recessed can or housing opening
– Example: If opening is 4″, a PAR38 (4.75″) won’t fit, so choose PAR30 (3.75″) instead
Step 2: Measure Depth Clearance
- From the socket base to the top of the housing
- PAR38: ~5.5″ long
- BR30: ~5.0″ long
- Shallow fixtures (<5″) need shorter bulbs
Warning: A bulb that’s too long can block airflow, trap heat, and shorten lifespan.
Step 3: Verify Base Type
Common bases:
– E26/E27: Standard screw base (most PAR, BR, A-shape)
– GU10/GU5.3: Pin base (MR16, some PAR16/20)
– Low-voltage: Requires transformer (e.g., PAR36/AR111)
Mismatched bases won’t install, so always confirm before buying.
Application-Based Size Guide
Match bulb size to your lighting goal.
Outdoor Security Lighting
- Best Bulb: PAR38
- Why: Rugged, weatherproof, long throw, flush fit
- Lumens: 1000–2000+
- Color Temp: 4000K–5000K (Daylight)
Avoid BR bulbs outdoors unless rated for wet locations because they are less durable.
Landscape and Pathway Lighting
- Best Bulb: PAR36 (AR111) or MR16
- Diameter: 4.5″ (PAR36) or 0.63″ (MR16)
- Low-voltage systems common
- Use warm white (2700K–3000K) for ambiance
PAR36 offers wide flood coverage; MR16 allows precise aiming.
Indoor Recessed Ceiling Floods
- Best Bulb: BR30 or BR40
- Soft light, even spread, minimal shadows
- Dimmable LED models available
- Pair with baffle trims to reduce glare
If fixture is shallow, consider PAR30 for flush fit.
Garage and High-Bay Lighting
- Best Bulb: Corn bulb or PAR38
- Corn bulbs: 10,000+ lumens, replace HID
- PAR38: Efficient, long-lasting, easy retrofit
Ensure fixture supports high heat and vibration.
Smart and Dimmable Flood Bulbs
Modern flood lights offer advanced control, but size still matters.
Smart PAR and BR Bulbs
Available in:
– PAR38 (outdoor security)
– BR30 (indoor ambient)
– MR16 (track/display)
Features:
– App control
– Scheduling
– Color tuning (CCT or RGB)
– Motion sensing (some models)
Ensure your fixture allows for heat dissipation because enclosed fixtures may reduce smart bulb lifespan.
Dimming Compatibility
Not all LED flood bulbs dim well:
– Check bulb and dimmer switch compatibility
– PAR and BR LEDs may require trailing-edge dimmers
– Poor pairing causes flickering or buzzing
Look for “dimmable” label and compatible dimmer list on packaging.
Avoid Common Sizing Mistakes
Even small errors can lead to poor performance.
Mistake 1: Ignoring Fixture Depth
Installing a BR30 in a 4″ deep can may cause the bulb to stick out, creating glare and a tripping hazard.
Fix: Measure depth first, and if under 5″, choose a shorter PAR bulb.
Mistake 2: Confusing PAR30 and BR30
Same diameter, different length and light quality:
– PAR30: flush, focused
– BR30: protruding, soft
Using the wrong one affects both fit and ambiance.
Mistake 3: Overlooking Base Type
Screw-in (E26) vs. pin base (GU10) are not interchangeable.
Tip: Take a photo of your old bulb before shopping.
Mistake 4: Assuming All Outdoor Bulbs Are Weatherproof
Not all BR or A-shape bulbs are rated for wet locations. Only use bulbs labeled “Wet Rated” outdoors.
PAR bulbs are generally safer for exposed fixtures.
Final Tips for Choosing Flood Light Bulb Dimensions
Getting the right size ensures safety, performance, and longevity.
Always Check the Label
Look for:
– Shape code (e.g., PAR38)
– Base type (E26, GU10, etc.)
– Diameter and length (if listed)
– Indoor/outdoor rating
When in Doubt, Remove the Old Bulb
Take it with you or measure it:
– Diameter at widest point
– Overall length
– Base width and pin spacing
Compare directly with new bulb specs.
Prioritize Fixture Compatibility
Even the brightest bulb fails if it doesn’t fit. Match:
– Diameter to housing opening
– Length to recess depth
– Base to socket type
– Rating to environment (wet/damp)
Consider Long-Term Maintenance
LED flood bulbs last 15,000–25,000 hours. Choose reputable brands with 3–5 year warranties to avoid frequent replacements.
Frequently Asked Questions About Flood Light Bulb Dimensions
What does the number on a flood light bulb mean?
The number represents the bulb’s diameter in eighths of an inch. For example, PAR38 equals 38 ÷ 8 = 4.75 inches wide. The exception is MR-series bulbs, where the number indicates millimeters (MR16 = 16mm).
What is the standard size of a flood light bulb?
The most common outdoor flood light bulb is PAR38 at 4.75 inches in diameter. For indoor recessed lighting, BR30 (3.75″) is the residential standard, while BR40 (5.0″) is common in larger spaces.
Are PAR38 and BR40 the same size?
No. PAR38 is 4.75 inches in diameter, while BR40 is 5.0 inches. They also differ in length and light quality, with PAR38 being shorter and more focused, and BR40 being longer and more diffused.
Can I use a BR30 instead of a PAR30?
Both have the same 3.75″ diameter, but BR30 is longer and produces softer, diffused light. If your fixture is shallow or you need focused beam control, stick with PAR30. If you want ambient lighting and the fixture is deep, BR30 works well.
How do I know what size flood light bulb fits my fixture?
Measure the fixture’s opening diameter and internal depth. Match these measurements to the bulb’s diameter and length specs. Also verify the base type (E26 screw or GU10 pin) matches your socket.
What size flood light bulb is best for outdoor security?
PAR38 is the industry standard for outdoor security lighting. Its 4.75″ diameter, rugged construction, and flush-mount design make it ideal for weatherproof housings and long-throw illumination.
Key Takeaways for Choosing the Right Flood Light Bulb Dimensions
Choosing the correct flood light bulb dimensions comes down to understanding the coding system and matching it to your fixture. PAR bulbs (especially PAR38) dominate outdoor and security applications because of their rugged build and flush fit. BR bulbs (BR30 and BR40) excel indoors where soft, diffused light is preferred. Always measure your fixture’s opening and depth before buying, verify the base type, and confirm the bulb’s wet-location rating if used outdoors.
Your next step: grab a tape measure, check your fixture’s dimensions, and use the size charts in this guide to find the perfect fit. With the right bulb in place, you’ll get reliable performance, optimal beam control, and years of maintenance-free illumination.
