Understanding your result
The headline is the enamel for the coats you chose, in quarts by brush or as a count of spray cans, and the sub-line adds the primer. The three figures show the surface, the quantity per coat and the primer.
The breakdown goes from the length including the gates, through the outline (length times height), the openness factor and the sides, to the surface to paint. For brushing it then shows the coverage, the enamel per coat and the total with the allowance, the cans to buy and the primer; for spray cans it shows the coverage per can and the whole cans for enamel and for primer. The arithmetic panel shows both.
By brush a railing is a quart-and-gallon job; by spray can it is a count that climbs quickly on anything solid. The fence stain calculator is the one for a wooden fence; the radiator paint calculator uses the same brush-or-can logic on radiators and metal panels indoors; and the garage door calculator handles a steel door, which is a solid panel rather than a railing.
How we calculate this
The outline is the rectangle the railing fills. The openness factor turns it into painted surface: for balusters at 4 in spacing, each 1 in bar has four faces, so a foot of railing has about 3 bars × 4 faces × 1 in × 36 in, roughly 3 sq ft per linear foot of a 3 ft railing against a 3 sq ft outline per side, which with the rails and posts comes to about 0.5 of the outline per side. Solid panels are the outline itself; open bars and cable are around 0.3. The fence guide covers the same reasoning for wooden pickets.
By brush, rust-inhibiting enamels cover about 350 sq ft per gallon on their data sheets, roughly 90 sq ft per quart, and the matching primers about 300; the 15% allowance is what dries on the brush and drips through the bars. By spray can, a 12 oz aerosol is rated at 12 to 20 sq ft and gets about 15 on a railing once the overspray through the gaps is counted, so the calculator divides the surface by 15 and rounds up to whole cans, ignoring the allowance because the 15 already includes the loss. The coverage rates table has the published figures for both forms.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Enamel by brush | 350 sq ft per gallon | Rust-inhibiting enamel data sheets (Rust-Oleum Stops Rust, Sherwin-Williams DTM Acrylic, Benjamin Moore Super Spec DTM): 300–400 |
| Primer by brush | 300 sq ft per gallon | Rust-inhibiting metal primer data sheets: 250–400 |
| Spray can | 15 sq ft per 12 oz can per coat | Aerosol cans quote 12–20 sq ft; 15 allows for overspray through bars |
| Openness | solid 1.0, balusters 0.5, open 0.3 | Site estimates from the bar geometry, explained above |
| Gate | 4 ft each | A standard garden gate |
| Coats | 2 enamel, 1 primer | Manufacturers’ recommendation |
| Allowance, brush | 15% | Brush loss on bars; a round figure |
Assumptions last reviewed October 7, 2026.
The calculator does not model rust removal, which is most of the work (wire brush, sanding or a rust converter), or the special primers aluminium and galvanized steel need. It does not count scrollwork, which can double the surface of an ornamental gate; estimate it as a solid panel. The drying guide covers oil-based enamels, which take a day between coats and a week to harden.
Two worked examples
A porch railing, brushed
40 ft of baluster railing plus a gate, 3 ft high, both sides, rust primer and two coats of enamel.
- Outline: (40 + 4) × 3 = 132 sq ft
- Surface: 132 × 2 sides × 0.5 = 132 sq ft
- Enamel: 132 ÷ 350 × 2 × 1.15 = 0.868 gallons, or 3.47 quarts, buy 1 gallon
- Primer: 132 ÷ 300 × 1.15 = 2.02 quarts, buy 3 quarts
A gallon of enamel and three quarts of primer, with a quart of each left for next year’s touch-ups. The surface equals the outline here because the balusters at 0.5 on two sides add up to one outline’s worth.
A solid steel gate panel, spray cans
A 10 ft solid gate, 4 ft high, both sides, primer and two coats from cans.
- Surface: 10 × 4 × 2 × 1.0 = 80 sq ft
- Enamel: 80 ÷ 15 × 2 = 10.67, buy 11 cans
- Primer: 80 ÷ 15 = 5.33, buy 6 cans
Seventeen cans is where spray cans stop making sense; a quart of enamel and a quart of primer by brush would do the same panel for a fraction of the cost. A short 8 ft open gate painted on one side with no primer, by contrast, is 7 sq ft and one can.
Where to find your inputs
Length and height. Along the top rail, and from the bottom rail to the top; posts add nothing worth counting. Gates count as 4 ft each.
Openness. Balusters or pickets at 4 in centres is the common residential railing; solid for sheet or close-spaced panels; open for wide bars, cable and mesh.
Sides. Both for a free-standing railing or fence; one for a panel against a wall.
Method. Brush (and a mitt or pad for the balusters) or 12 oz spray cans.
Common mistakes
- Painting over rust. It bubbles through in months. Wire-brush to bright metal and prime.
- Ordinary primer on aluminium or galvanized steel. It peels. Self-etching or galvanized-metal primer.
- Spray cans on a long open railing. Most of the paint goes through the gaps.
- One thick coat. Enamel sags on vertical bars; two thin coats.
- Recoating oil enamel too soon. A day between coats, or the second coat wrinkles the first.
- Forgetting the second side. Both faces of every bar, and the bars have four.
Questions people ask
- How much paint does a railing take?
- A 40 ft baluster railing with a gate, 3 ft high, painted on both sides, has about 132 sq ft of surface. Two coats of rust-inhibiting enamel by brush with a 15% allowance come to 3.47 quarts, so a gallon, plus three quarts of rust primer for one coat.
- How many spray cans for a railing?
- A 12 oz can covers about 15 sq ft per coat on a railing, after the overspray through the bars. A 10 ft solid gate panel 4 ft high, both sides, is 80 sq ft, so two coats take 11 cans and a primer coat 6 more. A short open gate takes one can.
- What does openness mean?
- How much of the railing's outline is actually metal. A solid panel is 1.0; balusters or pickets at 4 in spacing are about 0.5, counting both faces of each bar; widely spaced bars, cable or mesh are about 0.3. The surface is the outline times the openness, times two if both sides are painted.
- Do I need to prime metal before painting?
- Any rust, bare metal or a chalky old finish needs a rust-inhibiting primer after wire-brushing or sanding to bright metal, or the rust comes back through the enamel within a season. Sound, previously painted metal that has been scuffed can take enamel directly, and the calculator lets you untick the primer.
- Brush or spray cans?
- Brush for anything over about 20 ft or for open railings, where spray cans put most of the paint through the gaps. Spray cans for short gates, intricate scrollwork and touch-ups, where the finish is better and the waste is tolerable. Spray cans cost several times as much per square foot.
- Which paint for wrought iron?
- A rust-inhibiting enamel, oil-based or a direct-to-metal acrylic, over a rust-inhibiting primer. Aluminium needs a self-etching primer; galvanized steel needs to weather for a year or take a galvanized-metal primer, because ordinary primer peels from zinc.