By The Roofing Brief Team. Last reviewed: October 2026.
To measure gutter size, run a tape measure across the open top of the gutter from the front outer lip straight back to the fascia. That top-opening width is the size: about 5 inches means a 5-inch gutter, about 6 inches means a 6-inch gutter. K-style gutters can also be identified by girth (the width of the flat metal coil they were formed from), which is 11 3/4 inches for a 5-inch gutter, 15 inches for a 6-inch and 18 inches for a 7-inch, per Lynch Aluminum’s gutter coil specifications. Take three readings along the run and average them, because dents and seams distort a single reading.
Two jobs get mixed up: identifying the size you already have, and sizing a new gutter to your roof’s drainage area. The tape reading and the downspout settle the first; the second uses roof area, a pitch factor and local rainfall intensity, as set out in This Old House’s gutter and downspout sizing guide (updated April 20, 2026). For the full capacity table and how each size performs by material and rainfall, see the gutter sizes and capacity reference.
What “gutter size” actually measures
Gutter size refers to the width of the trough opening at the top, measured from the front lip back to the fascia side, not the depth or the length of the run. The three residential sizes are 5-inch, 6-inch, and less commonly 7-inch. That single number sets how much water the trough can carry and which downspout and guard products fit it: United Gutter Guard, for example, sells its standard guard for gutters 5 inches wide or less and its super size for 6 inches or more, measured the same way.
Two other numbers get confused with size. Depth is the vertical height of the trough (usually 3 to 4 inches on K-style) and does not define the size. Girth is the flat width of the coil stock before it is roll-formed: Lynch Aluminum sells .027 and .032 inch thick coil in 11 3/4 inch widths for 5-inch gutters and 15 inch widths for 6-inch gutters.
How to measure gutter size step by step
Measuring an installed gutter takes a tape measure, a stable ladder, and two minutes. The goal is the top-opening width, read at the widest point of the mouth. The decorative front face height and the back leg alone are each 3 to 4 inches or so on K-style, so neither equals the nominal size.
- Set the ladder safely. Place it on firm ground at the eave with the foot one quarter of the working length out from the wall (the 4 to 1 rule in OSHA 1926.1053(b)(5)(i)), so a ladder touching at 16 feet stands 4 feet out. Have a spotter if the gutter sits above the first story.
- Find the top opening. Look straight down into the trough. The opening runs from the front outer lip to the point where the back of the gutter meets the fascia board, about 4 1/4 to 5 inches across on a 5-inch K-style.
- Lay the tape across the mouth. Hook the tape on the front lip and pull it straight back to the fascia, keeping it level and not dipping into the trough, and read it to the nearest 1/8 inch.
- Read the width. About 4 to 5 inches indicates a 5-inch gutter, about 5 to 6 inches indicates a 6-inch gutter, and roughly 7 inches indicates a 7-inch gutter.
- Average three spots. Take readings near a downspout, at a midpoint, and near a corner, then average the three numbers, since a dent or a seam can throw off one reading by enough to land between sizes.
If you only need the size to order gutter guards or a downspout adapter, the top-opening width is the only number you need: United Gutter Guard, for example, splits its range at 5 inches or less versus 6 inches or more. If you are replacing the gutters, you also need the linear-foot length of every run, measured along the fascia, and the count of corners and downspouts.
| If your measurement reads | You most likely have | Confirm with |
|---|---|---|
| Top opening 4 1/4 to 5 in | 5-inch K-style | 2 by 3 inch downspout; 11 3/4 in girth |
| Top opening 5 to 6 in | 6-inch K-style | 3 by 4 inch downspout; 15 in girth |
| Top opening about 7 in | 7-inch K-style (often commercial) | 18 in girth (Lynch 7-inch coil) |
| Semicircular trough, no flat back | Half-round | Round downspout; size by diameter, not girth |
| Reading falls exactly between two sizes | Undetermined | Downspout size, then a second reading away from seams |
How to tell 5-inch from 6-inch gutters
A 5-inch K-style gutter has a top opening close to 5 inches and a 6-inch K-style opens near 6 inches. Published capacity figures differ by source: Englert says 6-inch K-style gutters handle about 40% more water than 5-inch, Gutter Supply’s half-round vs K-style comparison lists 1.4 gallons per foot for 5-inch K-style and 2.0 for 6-inch, and This Old House rates them at 5,520 and 7,960 adjusted square feet of drainage area. When a tape reading lands between the two, use the confirming clues below.
| Clue | 5-inch K-style | 6-inch K-style | Source |
|---|---|---|---|
| Top opening (front lip to fascia) | About 4.25 to 5 in | About 5 to 6 in | Tape reading |
| Coil girth (flat stock width) | 11 3/4 in | 15 in | Lynch Aluminum |
| Water held per linear foot | 1.4 gal | 2.0 gal | Gutter Supply |
| Rated drainage area (adjusted sq ft) | 5,520 | 7,960 | This Old House |
| Paired downspout and its rated area | 2 by 3 in, about 600 sq ft | 3 by 4 in, up to 1,200 sq ft | This Old House |
| Typical use | Most single-family homes | Large or steep roof planes, heavy rain | Englert |
The downspout is often the fastest tell from the ground. A small rectangular 2-by-3-inch downspout almost always means 5-inch gutters, while a larger 3-by-4-inch downspout signals 6-inch. If you cannot reach the trough at all, photograph the downspout against a ruler or a standard credit card (3.37 inches wide) for scale.
The girth method for K-style gutters
Girth is the flat width of the metal coil a gutter is formed from, and reading it lets you confirm size without measuring the exact opening. Lynch Aluminum lists 11 3/4 inch coil for 5-inch gutters, 15 inch for 6-inch and 18 inch for 7-inch, in 3105 alloy at .027 or .032 inch thickness. If you have an offcut, a loose section, or access to the back, measuring girth is more forgiving than reaching into an installed trough.
- Find a flat sample or trace the profile. Use an unformed offcut, or press a strip of paper into the full contour of the gutter from the back top edge to the front lip, including the hem at the lip.
- Flatten and measure. Unroll the sample or the paper trace and measure its total width in inches.
- Match the girth. Around 11 3/4 to 12 inches indicates a 5-inch K-style, around 15 inches a 6-inch K-style, and around 18 inches a 7-inch.
Half-round gutters use different coil widths and are identified by shape, not girth, which is covered next.
K-style vs half-round: identify the profile first
Before you trust a size number, identify the profile, because the same nominal size holds different volumes in each shape. K-style has a flat back and flat bottom with a decorative ogee front that resembles crown molding. Half-round is a simple semicircular trough, sized by its diameter. Englert lists 5-inch and 6-inch as the standard K-style sizes, and K-style is the default on most American homes; half-round shows up on older, historic, or high-end houses.
Profile matters for capacity. Gutter Supply lists a 5-inch half-round at 1.2 gallons per foot against 1.4 for a 5-inch K-style, and This Old House rates a 5-inch half-round at 2,500 adjusted square feet of drainage against 5,520 for a 5-inch K-style. For a full breakdown of how the two shapes compare on capacity and cost, see K-style vs half-round gutters.
What size gutters do I need? Sizing to roof drainage area
Sizing a new gutter starts from the roof drainage area feeding each run, not the size you happen to have now. The method This Old House publishes multiplies the roof plane’s footprint by a pitch factor and by local rainfall intensity in inches per hour to get an adjusted square footage, then matches that number to a gutter’s rated capacity: up to 5,520 for 5-inch K-style and up to 7,960 for 6-inch K-style.
- Measure each roof plane. Multiply the length by the width of every plane that drains into the gutter run to get its footprint in square feet; a 40 by 20 foot plane is 800 square feet.
- Apply the pitch factor. This Old House uses 1.05 for 4-in-12 to 5-in-12, 1.1 for 6-in-12 to 8-in-12, 1.2 for 9-in-12 to 11-in-12, and 1.3 for 12-in-12 or steeper.
- Adjust for rainfall. Multiply by your area’s maximum rainfall intensity in inches per hour, which you can look up by address in the NOAA Atlas 14 Precipitation Frequency Data Server.
- Match to a size. At or under 5,520 adjusted square feet, a 5-inch K-style works; from 5,520 to 7,960, choose 6-inch; above 7,960, add downspouts or step up to 7-inch.
| Worked example (40 by 20 ft plane, 7-in-12 pitch) | Value |
|---|---|
| Footprint | 800 sq ft |
| Times pitch factor 1.1 | 880 sq ft |
| Times 6 in/hr rainfall intensity | 5,280 adjusted sq ft: 5-inch K-style is enough |
| Times 7 in/hr rainfall intensity | 6,160 adjusted sq ft: step up to 6-inch K-style |
Slope and downspout count matter as much as trough width. This Old House gives three rules: pitch gutters about 1/4 inch per 10 feet toward the outlet, provide about 1 square inch of downspout cross-section per 100 square feet of roof, and give any run of 60 feet or longer at least two outlets. See gutter slope and drop per foot for the pitch side, and downspout placement and sizing for how many outlets a run needs.
Common measuring mistakes
Most size errors come from measuring the wrong dimension. Reading the 3 to 4 inch front-face height instead of the top opening, or measuring a dented section, produces a number that does not match any standard size. When in doubt, confirm with a second clue such as the 2 by 3 or 3 by 4 inch downspout footprint or the 11 3/4 or 15 inch coil girth.
- Measuring depth, not width. The 3-to-4-inch vertical height is not the size; the top opening is.
- Trusting one reading. Dents, seams, and forming drift skew a single spot; average three.
- Ignoring the profile. A 5-inch half-round and a 5-inch K-style are not interchangeable on capacity.
- Forgetting the downspout check. The 2-by-3 versus 3-by-4 outlet is the quickest ground-level confirmation.
Frequently asked questions
How do I measure my gutter size?
Lay a tape measure across the top opening of the gutter, from the front outer lip straight back to the fascia. That width is the size. About 5 inches is a 5-inch gutter and about 6 inches is a 6-inch gutter. Take three readings along the run and average them, since dents and seams distort single spots.
How do I tell if I have 5-inch or 6-inch gutters?
Measure the top opening, or check the downspout. A 2-by-3-inch downspout almost always pairs with 5-inch gutters and a 3-by-4-inch downspout with 6-inch. Englert says 6-inch K-style gutters carry about 40% more water than 5-inch, and This Old House rates them at 7,960 versus 5,520 adjusted square feet of drainage area.
What is the girth of a gutter?
Girth is the flat width of the metal coil the gutter is roll-formed from, measured before shaping. Lynch Aluminum sells 11 3/4 inch coil for 5-inch K-style gutters, 15 inch coil for 6-inch and 18 inch coil for 7-inch. Measuring an offcut’s girth confirms size without reaching into an installed trough.
What size gutters do I need for my roof?
Size to the roof drainage area, not the gutters you have now. Multiply each roof plane’s footprint by a pitch factor of 1.05 to 1.3 and by your maximum rainfall intensity in inches per hour, per This Old House’s method. At or under 5,520 adjusted square feet a 5-inch K-style works; up to 7,960 calls for 6-inch.
Do I measure gutter depth or width to find the size?
Width, meaning the top opening from front lip to fascia. Depth is the 3-to-4-inch vertical height of the trough and does not define the nominal size. Measuring depth by mistake is the most common reason a homeowner gets a number that matches no standard gutter size.
How do I measure gutter length for a replacement?
Run a tape along the fascia for each straight section and add the sections for total linear feet. This Old House says runs of 60 feet or longer need at least two outlets, and suggests about 1 square inch of downspout cross-section per 100 square feet of roof. Length, downspout count and a slope of about 1/4 inch per 10 feet set drainage as much as trough width does.