Sources & formulas

Every calculator on WindowCalcs rests on established fenestration geometry and heat transfer plus stable window/door conventions. Unlike topics with a time dependency, “verification” here is mathematical: each formula is tested against known values and is therefore correct for good. Here are the technical foundations by area.

Identities & geometry

  • United inches: UI = width + height — the industry ordering/pricing size.
  • Glass area: width × height ÷ 144 (144 in² = 1 ft²).
  • Rough opening: unit size + 2 × shim gap (window ~½ in/side → +1 in each dimension; pre-hung door +2 in wide, +2½ in tall).
  • R-value: R = 1 ÷ U-factor (and U = 1 ÷ R) — the reciprocal identity.
  • Egress: pass ⇔ clear area ≥ the labeled IRC minimum AND clear width ≥ 20 in AND clear height ≥ 24 in AND sill ≤ 44 in.
  • Energy saving: annual = area × (U_old − U_new) × HDD × 24 ÷ 1000 ÷ furnace efficiency × your energy price; payback = project cost ÷ annual saving.
  • Quantities: window-to-wall ratio = window area ÷ (perimeter × wall height); trim = Σ opening perimeters ÷ 12, × (1 + waste).

Convention typicals (labeled)

  • United-inches convention: suppliers order/price by width + height — labeled.
  • Standard shim / jamb gaps: window ~½ in per side; pre-hung door +2 in W / +2½ in H — labeled, confirm the spec sheet.
  • IRC R310 egress minimums: clear opening ≥ 5.7 sq ft (≥ 5.0 at a grade floor), width ≥ 20 in, height ≥ 24 in, sill ≤ 44 in — labeled published snapshot, confirm local code & the AHJ.
  • ENERGY STAR 7.0 & IECC 2021 U/SHGC by climate zone: e.g. ENERGY STAR Northern U ≤ 0.22, North-Central U ≤ 0.25; IECC zone 5 U ≤ 0.30 — labeled published snapshots, confirm the current criteria.
  • Typical U-factor & SHGC by frame × glazing: e.g. double-pane low-E + argon ~U 0.30 (R 3.33), triple-pane ~U 0.18 (R 5.56) — labeled, use the NFRC label.
  • Cost bands, material lifespans, install multipliers, contingency/waste ~5–10%: labeled planning typicals.

Cost tools

  • Every cost tool uses the prices you enter ($/window, $/door, labor $, add-on $, disposal $) — no material or labor price is stored, so the site needs no maintenance. Cost bands are a labeled sanity guide.
  • Cost: total = (count × your $/unit + labor + add-ons(full-frame vs insert, custom size, trim/casing, disposal, two-story/access, lead-safe) − discount) ×(1 + contingency).
  • Quote check: derived $/window = quote ÷ count, compared to the labeled band for the material — a sanity flag, not a bid.

Material and labor prices, product data-sheet values, code and local rules vary by place and change over time — always measure each opening and confirm sizes, egress and energy ratings against the exact product and your local building code (AHJ) and the NFRC label, allow extra for custom sizes and waste, defer structural headers, whole-building heat-load and code certification to a certified professional, and get itemized written quotes from a licensed, insured window/door installer before you commit.