ForHosting KIT · Developer Utilities

Primer calculator

The primer calculator estimates how many gallons of primer a wall or ceiling project requires.

● BetaFree · in your browser

Use it from WebAPIEmailTelegramApp soon

Enter the total surface area in square feet, the coverage rate printed on the primer label in square feet per gallon, and the planned number of coats. The result shows both the exact calculated volume and the whole number of gallons to buy, making it useful for planning a single room, a ceiling refresh, a renovation, or a larger professional painting job.

Measure the surface area before estimating primer

Begin with the total area that will actually receive primer. For a rectangular wall, multiply width by height, then repeat for every wall and add the results. For a flat rectangular ceiling, multiply room length by room width. When doors, large windows, cabinets, or permanently covered sections will not be primed, subtract their areas if the deduction is meaningful. Small openings can be left in the total as a practical allowance for spills, roller loading, and surface variation. Keep every measurement in feet so the final area is in square feet, because this calculator expects a coverage rate expressed as square feet per gallon. If another tool or drawing already gives you the net paintable area, enter that figure directly. The calculator does not infer wall area from room dimensions and does not decide which openings should be excluded. Careful measurement matters most on large projects, where even a modest percentage error can change the number of cans you need to purchase.

Use the coverage rate printed for your primer

Coverage rate means the area that one gallon can cover with one coat under the manufacturer’s stated conditions. Read the product label or technical data sheet and enter its square-feet-per-gallon figure rather than relying on a generic assumption. Porous drywall, bare masonry, raw wood, repairs, texture, and application technique can all reduce real coverage. A previously sealed, smooth surface may come closer to the published rate. If a label provides a range, choosing the lower coverage value gives a more conservative estimate because dividing by a smaller rate produces more gallons. The coats field defaults to one, which suits many routine priming projects, but some stain-blocking, color-change, or highly absorbent surfaces may require another coat according to the product instructions. The calculation multiplies surface area by coats and divides that total coated area by coverage per gallon. Units must match: this page uses square feet and US gallons, so do not enter square meters per liter without converting them first.

Interpret exact gallons and gallons to buy

The result includes exact gallons, which is the direct mathematical estimate, and gallons to buy, which rounds that estimate upward to a whole gallon. The exact value is useful for comparing products, estimating cost, or combining several projects that use the same primer. The rounded purchase quantity is a straightforward shopping figure when primer is available only in one-gallon cans. Before buying, check the actual container sizes sold locally; a combination of five-gallon pails, gallons, and quarts may be more economical than the displayed whole-gallon count. Rounding up also creates some allowance, but it is not a formal waste factor and cannot predict absorption, overspray, application loss, damaged material, or leftover primer trapped in equipment. Professional jobs may add a project-specific contingency after considering substrate condition and application method. The estimate is based entirely on the values entered, so it should support—not replace—the primer manufacturer’s preparation, application, drying, and recoating directions. API requests cost $0.002, while the browser calculation uses the same deterministic formula.

What you can do with it

Prime new drywall

Estimate the gallons needed to seal measured walls and ceilings before applying finish paint.

Plan a ceiling project

Convert the combined ceiling area and the selected product coverage into a practical purchase quantity.

Compare primer products

Use each label’s coverage rate to compare required volume before evaluating package prices.

FAQ

What does the calculator cost?

The API price is $0.002 per request, and the calculator can also run in the browser.

Which area and volume units does it use?

Enter area in square feet and coverage in square feet per US gallon; the result is in US gallons.

Should I subtract doors and windows?

You may subtract large areas that will not be primed, although leaving small openings included can provide a modest practical allowance.

Why is gallons to buy higher than exact gallons?

The purchase figure rounds up to the next whole gallon so the mathematical requirement is not rounded down.

Does the estimate include waste?

No explicit waste percentage is added. Actual use can increase with porous surfaces, texture, overspray, spills, and application technique.

For developers — API access

Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.

API endpoint

POSThttps://api.kit.forhosting.com/home/primer-paint

Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.

Call it from your stack

curl -X POST https://api.kit.forhosting.com/home/primer-paint \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"area":1200,"coverage_rate":300}'

const res = await fetch("https://api.kit.forhosting.com/home/primer-paint", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "area": 1200,
    "coverage_rate": 300
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/home/primer-paint",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "area": 1200,
    "coverage_rate": 300
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/home/primer-paint", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"area":1200,"coverage_rate":300}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"area":1200,"coverage_rate":300}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/home/primer-paint", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "area": 1200,
  "coverage_rate": 300
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "home.primer_paint",
  "status": "queued",
  "_links": {
    "result": "/tasks/tsk_…/result"
  }
}

The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.

Pricing

Per request$0.002

Published price — no tokens, no invented credits. A failed task is never charged.

Errors

HTTP	Code	Meaning
`401`	`unauthorized`	Missing or invalid API key.
`402`	`insufficient_balance`	Your balance doesn't cover the task price.
`404`	`unknown_type`	That task type doesn't exist.
`429`	`rate_limited`	Too many requests. Use the webhook instead of polling.

Read the full KIT documentation →