The Free Construction Calculators: What They Estimate

What each of the eleven calculators and the cost estimator work out, the waste factors and coverage rates built in, how results round, and why nothing is saved to your project.

What these are

OwnerDraw publishes eleven quantity calculators plus a whole-build cost estimator. They are public web pages: no sign-in, no subscription, no AI credits, and nothing is charged.

  • **All Calculators** — `/construction-calculators`, all eleven in one page with tabs.
  • One page per calculator, for example `/roofing-calculator` or `/stair-calculator`.
  • **Cost Estimator** — `/estimator`, the whole-project cost calculator, which works differently (see the end).

Start here

1. Open **All Calculators** and pick the trade you are pricing.
2. Fill the boxes. Every unit is printed beside the box — **ft**, **in**, **sq ft**, **LF**, **pcs**, **%**, **$**.
3. Select **Calculate**.
4. Read the **Results** tiles and the note underneath them. The note carries the code references and the "verify with your supplier" caveats.

Nothing calculates as you type — results appear only after **Calculate**, and change only when you select it again.

What each one estimates

| Calculator | You enter | You get |
|---|---|---|
| **Roofing** | Building length × width (or 2D plan area, plus optional ridge and eave lengths), pitch, waste % | 3D roof area, squares, shingle bundles, underlayment rolls, pitch multiplier — plus ridge cap bundles and drip edge LF when ridge/eave lengths are known |
| **Wall Framing** | Wall linear feet, spacing, wall height, doors, windows, corners, T-walls | Total studs, base layout studs, corner/T extras, opening extras, plate LF and 16' pieces, cripples |
| **Concrete Slab** | Length, width, thickness (in), waste % | Cubic yards, cubic feet, area, 80 lb and 60 lb bag counts, ready-mix trucks |
| **Footings** | Linear feet, width (in), depth (in), waste % | Cubic yards, bag counts, ready-mix trucks |
| **Drywall** | Room length, width, ceiling height, doors, windows, include-ceiling tick | Recommended board size, why, net area, sheet count, joint compound, tape, screws, corner bead |
| **Paint** | Room length, width, height, coats, doors, windows, include-ceiling tick | Wall gallons, ceiling gallons, total finish, primer, net areas |
| **Flooring** | Room length, width, material, pattern | Room area, order quantity with waste, waste allowance, approximate boxes, squares |
| **Board Feet** | Nominal thickness, nominal width, length, quantity | Nominal board feet per piece and total, plus nominal cubic inches per piece |
| **Stairs** | Total floor-to-floor rise (in), tread depth (in) | Risers, actual riser height with an IRC pass/warn flag, treads, total run, stringer length |
| **Insulation** | Area, location, frame or climate type | A commonly used R-value, insulation type, area including 10% waste, estimated packages (from area and package coverage only) |
| **Builder's Insurance** | Construction value, duration in months, state, structure type, project type, W-2 employees | Builder's Risk premium range, required coverage, General Liability range, whether Workers' Comp applies |

The assumptions built in

These are baked into the results whether or not you see them on screen.

  • **Waste.** Roofing, slab and footings let you set the percentage (defaults 10%, 8%, 8%). Drywall adds a fixed **10%**, framing a fixed **5%** buffer, insulation a fixed **10%**. Flooring uses the material and pattern to pick 10–20%. Paint adds no waste at all.
  • **Openings.** Drywall and paint deduct **21 sq ft per door** and **15 sq ft per window** — regardless of the sizes on your plans.
  • **Coverage rates.** Wall paint 350 sq ft/gal, ceiling 300, primer 250. Joint compound 1 gal per 100 sq ft. Insulation 40 sq ft per package. Underlayment 400 sq ft per roll. Ridge cap 25 LF per bundle.
  • **Products.** Shingles at 3 bundles per square. Concrete at 45 × 80 lb bags or 60 × 60 lb bags per cubic yard, 10 CY per ready-mix truck. Flooring boxes at roughly 20 sq ft. All of these vary by manufacturer.
  • **Shapes.** Roofing assumes a simple gable. Drywall and paint assume one rectangular room. Footings assume a continuous strip.
  • **Roofing from plan area.** Plan area alone tells the calculator nothing about the shape of your house, so in **2D Sq Ft from Plans** mode ridge cap and drip edge are left out unless you type in a **Ridge Length** and an **Eave + Rake Length**. Roof area, squares, shingles and underlayment come from the area and pitch and appear either way. In **Length × Width** mode the ridge is taken as the length and the perimeter as (length + width) × 2 — right for a simple rectangular gable, not for an L-shaped or hipped roof.
  • **Board feet** are *nominal* by definition — a 2×4 counts as 2" × 4", because that is the unit lumber is priced and sold in. The "Nominal Cubic Inches" tile follows the same nominal sizes, so it is not the physical volume of the milled board (a real 2×4 is 1.5" × 3.5").
  • **Insulation packages** come from your area plus **10% waste**, divided by **40 sq ft per package**, rounded up — nothing else. So 800 sq ft becomes 880 sq ft, and 880 ÷ 40 gives **22 packages**. Changing the R-value or the frame type does not change the count, and neither the count nor the R-value shown proves code compliance — check your local code and the coverage printed on the product you buy.
  • **Insurance** applies a 1.5% base Builder's Risk rate with state, structure and renovation multipliers, then shows a range around it, and a General Liability range around $1,500 a year adjusted by state.

Reading the results

  • **Whole units are rounded up.** Bundles, bags, sheets, gallons, rolls, packages, trucks, risers and squares are always rounded up to the next whole one — you cannot buy 4.2 bags.
  • **Areas and volumes keep decimals** or round up to the next whole unit; cubic yards show two decimals.
  • Where a tile shows a "raw" figure underneath (for example *8.43 raw squares*), that is the unrounded number.
  • Stair results include a code check: **✓ IRC compliant** or **⚠ Exceeds IRC max 7-3/4"**. That is arithmetic against the published limit, not an inspection.
  • Insurance results are ranges, in whole dollars, and are not quotes.

Are results saved or connected to my project?

No, in every sense that matters:

  • Results are not saved. Leave the page or reload it and they are gone. There is no history.
  • There is no copy, download, email or print action. To keep a figure, write it down or take a screenshot.
  • Nothing is written to a project, Budget line, allowance, takeoff or expense — even if you are signed in. To use a number in your build, type it into the Budget line yourself.
  • No order is placed with anybody, and no supplier sees your inputs.

What do I do next?

| You want to… | Do this |
|---|---|
| Price a quantity into your budget | Calculate it here, then enter it on the **Budget** line as Qty and $/Unit yourself |
| Measure quantities off your actual plans | Use **Takeoff** inside OwnerDraw instead — it records where each measurement came from |
| Sanity-check a contractor's quantity | Run the calculator with the same inputs and compare, then ask about the difference |
| Estimate the whole build | Use the **Cost Estimator** at `/estimator` |
| Get insurance figures you can rely on | Take the checklist under the insurance calculator to three agents who write construction policies |

Common questions

**Do I need an account?** No. These pages are open to anyone and use no credits.

**Is this an order or a quote?** Neither. Order quantities come from your supplier's takeoff, and prices come from quotes.

**Will it match my supplier's list?** Not exactly. Bundle, bag and box coverage vary by product, and the calculators assume simple shapes.

**Can I trust the code references?** They are the published minimums as a guide — IRC stair limits, IECC insulation R-values, footing width rules. Your local building department has the final say, and some jurisdictions are stricter.

**Does the stair check mean my stairs pass inspection?** No. It compares two numbers you typed against the code limit. Nothing about headroom, landings, handrails or framing is checked.

**What about the Cost Estimator?** It is a different tool: you choose New Build, Land & Build or Renovation, add size, storeys, bedrooms, bathrooms, spec level and extras, and it returns a total with a percentage breakdown by category (Interior Finishes, Major Systems, Framing, and so on). It uses the same rules as the quantity calculators: no sign-in, no saving, no connection to your project or budget, and it is not a lender's figure or a builder's quote.

For professional builders

Treat these as order-of-magnitude checks and client conversation tools. The fixed 21/15 sq ft opening deductions, single-room geometry, 3-bundle-per-square shingle assumption and generic package coverage make them unsuitable as a purchase takeoff. For anything you will buy against, use **Takeoff** on the real drawings, then confirm with the supplier's own quantity list. The insurance figures are rate-of-thumb multipliers, not underwriting.