About WindowCalcs
WindowCalcs is an independent project with one goal: to gather the everyday calculations homeowners, DIYers and small contractors reach for when windows and exterior doors go in — sizing & quantity, cost estimators, types/frames/glass, energy efficiency, exterior doors, and installation/planning — in one focused, free, no-signup hub with transparent formulas, so you can budget a window or door project and sanity-check an installer’s quote.
Who is behind it
Author and curator
To be clear about credentials: I am the author and curator of this site — not a licensed window or door installer, a glazier, an architect or any trade professional, and I claim no trade credential. What I bring is relevant and real: building deterministic online calculators (open-source Python projects) and electronic-engineering training (Systems Theory, Sapienza University of Rome). Window sizing is plane geometry (united inches = width + height; glass area = width × height ÷ 144; rough opening = unit size + shim gaps) and window energy performance is a steady-state conduction problem (R-value = 1 ÷ U-factor; heating-season loss = area × U × HDD × 24; SHGC governs solar gain) — the same conductance/flow reasoning I apply in systems and electronics, so the math of these calculators is squarely in my competence, while installation judgement (opening prep, flashing, structural headers, local code, AHJ egress sign-off) is explicitly deferred to licensed pros, the NFRC label and manufacturer data.
Our principle: transparent & durably correct
Every calculator shows its formula, a worked example and a reference table. The tools rest only on timeless fenestration geometry and heat transfer (united inches = w + h; area = w × h ÷ 144; rough opening = size + shim gaps; R = 1 ÷ U; egress pass/fail against the labeled IRC minimums; annual saving = area × ΔU × HDD × 24 ÷ 1000 ÷ efficiency × price; payback = cost ÷ savings; cost = count × your $/unit + labor + add-ons, ×(1 + contingency)) and stable, labeled conventions (united-inches sizing, shim gaps, IRC R310 egress, ENERGY STAR / IECC U/SHGC, U/SHGC by frame × glazing, cost bands). There are deliberately no live material or labor prices, no regional cost indexes, no product catalog and no contractor directory — cost tools use the prices you enter — so the results stay valid over time.
Correctness is checked against known reference values (see the methodology and the numeric self-check). The formulas and their basis are documented under Sources & formulas. All results are planning estimates and sizing / quantity / egress-screening / energy guides: get itemized written quotes from licensed, insured window/door installers, measure each opening (smallest of three) and confirm sizes, egress and energy ratings against the exact product, and allow extra for custom sizes and waste. Structural headers for enlarged openings, whole-building heat-load / HVAC, wall/attic insulation, professional energy audits and code certification are out of scope and are for local code and a certified professional. Questions? Use the contact page.