Methodology & Sources
Last updated
How a calculator gets built
- Formula first. We start from physics or geometry (trigonometry for roof pitch, Ohm's law for electrical, heat transfer for HVAC) and write down every assumption.
- Tested code. The math lives in a small function with no connection to the page layout. Each one has at least five automated tests, including zero, negative, very large and unit-conversion cases.
- Hand check. Before publishing, we work through at least one example by hand and compare the result with the calculator and with a public reference.
- Show the work. Every page includes the formula, a worked example and the sources, so you can verify the result yourself.
All calculations run in your browser. Costs are always shown as low, typical and high ranges, with the year of the data, never as a single exact price.
Codes and standards
We cite building codes and standards by section number, such as the International Residential Code (IRC) and the National Electrical Code (NEC), and link to the publisher. We don't reproduce copyrighted code tables. Where a tool needs values like wire ampacity or voltage drop, we compute them from published physical properties (for example, the resistivity of copper and aluminum) and tell you to confirm against the code edition your jurisdiction uses.
HVAC sizing tools give a simplified load estimate. They are not a replacement for a full ACCA Manual J calculation, which a qualified HVAC contractor should perform before you buy equipment.
State data
State pages and preloaded calculators use one shared dataset covering the 50 states and Washington, DC, rebuilt by a script from public sources. Each field records where it came from and when:
| Data | Source | Period | Retrieved |
|---|---|---|---|
| Median hourly wage (USD): roofers 47-2181, HVAC mechanics 49-9021, electricians 47-2111 | U.S. Bureau of Labor Statistics, Occupational Employment and Wage Statistics (OEWS) | May 2025 | 2026-10-30 |
| Average residential electricity price (cents/kWh) | U.S. Energy Information Administration, Electric Power Monthly, Table 5.6.B | August 2026 year to date | 2026-10-30 |
| Annual heating and cooling degree days, base 65 °F (population-weighted statewide; AK, DC, HI from airport station normals) | NOAA NCEI, nClimDiv statewide degree days | 1991–2020 average | 2026-10-30 |
| Hail reports per year (≥ 1 in and ≥ 2 in); risk tier from ≥ 2 in reports per 10,000 sq mi | NOAA Storm Prediction Center, Severe Weather Database | 2016–2025 | 2026-10-30 |
| Design temperatures (°F) at the nearest hourly-normals station to a reference city: cooling = highest Jun–Sep hourly 90th percentile, heating = lowest Dec–Feb hourly 10th percentile, dew point = highest Jun–Sep hourly normal | NOAA National Centers for Environmental Information, U.S. Climate Normals 1991–2020 (hourly) | 1991–2020 hourly normals | 2026-10-30 |
| 5-minute rainfall intensity (in/h), 10- and 100-year recurrence, at a reference city (partial-duration series) | NOAA National Weather Service, Hydrometeorological Design Studies Center, Precipitation Frequency Data Server (Atlas 14) | NOAA Atlas 14 | 2026-10-30 |
| IECC climate zones present; primary = zone of largest population center | 2021 International Energy Conservation Code, Figure R301.1 (via U.S. DOE Building Energy Codes Program) | IECC 2021 | 2026-10-30 |
| Land area (sq mi) | U.S. Census Bureau, 2010 Census State Area Measurements | 2010 Census | 2026-10-30 |
Notes on specific fields
- Wages are state median hourly wages for employees in each trade. A contractor's billed labor rate is higher because it covers insurance, overhead, equipment and profit. Our cost tools apply a stated markup to the wage rather than presenting the wage as the price.
- Degree days are 1991–2020 averages of population-weighted statewide values. NOAA doesn't publish a statewide series for Alaska, Hawaii or Washington, DC, so for those we use the 1991–2020 normal at the main airport station (Anchorage, Honolulu and Reagan National).
- Hail risk is our own tier, based on reports of hail 2 inches or larger per 10,000 square miles per year over the last ten complete years. We use large hail because reports of smaller hail track how many people live in an area as much as the weather itself. Very small areas (DC, Rhode Island, Delaware) are pooled with their neighbors to avoid noisy numbers. Tiers: low (under 0.75), moderate (0.75–2.5), high (2.5–5), very high (5 or more).
- Design temperatures are our own definition, not ASHRAE's: the highest hourly 90th-percentile temperature from June through September (cooling), the lowest hourly 10th-percentile temperature from December through February (heating), and the highest hourly dew point normal in summer, all from NOAA's 1991–2020 hourly normals at the station nearest a major city in each state. They land within a few degrees of the design values HVAC engineers use, but local conditions can differ.
- Rainfall intensity is the 5-minute, 10-year and 100-year intensity from NOAA Atlas 14 at the same reference city. Atlas 14 doesn't cover Oregon or Washington, so the gutter calculator asks for a local value there.
- IECC climate zones list every zone found in the state; the primary zone is the one covering the largest population center. Zones are assigned by county, so check your county if you live near a boundary.
Heating and cooling loads
Our HVAC sizing tools use a simplified whole-house heat balance, not ACCA Manual J. Each story is modeled as a square box. Cooling adds conduction through walls, windows and the ceiling (with the attic 35 °F above outdoors), sun through glass at 70 BTU/h per sq ft × SHGC × exposure, air leakage (sensible and latent), 230 sensible and 200 latent BTU/h per person, and 1200 BTU/h for appliances. Heating adds slab-edge loss and 30% more air leakage, with no credit for sun or people. Ducts outside the conditioned space add 15%. Indoor conditions are 75 °F at 50% relative humidity in summer and 70 °F in winter.
| Wall U-value (BTU/h·ft²·°F) | 0.2 / 0.08 / 0.05 |
|---|---|
| Ceiling U-value | 0.1 / 0.033 / 0.021 |
| Slab or floor edge (BTU/h·ft·°F) | 0.75 / 0.55 / 0.45 |
| Air changes per hour (drafty / average / tight) | 0.7 / 0.45 / 0.25 |
| Sun exposure factor (shaded / some / full) | 0.6 / 1 / 1.25 |
| Single pane: U-value / SHGC | 1.1 / 0.8 |
| Double pane, clear: U-value / SHGC | 0.5 / 0.65 |
| Double pane, low-E: U-value / SHGC | 0.3 / 0.35 |
Equipment is the smallest nominal size covering at least 95% of the load. Heat pump output is modeled as a straight line from 100% at 47 °F to 60% at 17 °F for standard units and 75% at 5 °F for cold-climate units. Cooling energy uses full-load hours of 24 × cooling degree days ÷ (design temperature − 65 °F).
Cost estimates
Every cost we show is a low, typical and high range in 2026 dollars. Labor is priced from the BLS median wage for the trade in your state, multiplied by a billing factor for payroll taxes, workers' comp, insurance, overhead and profit. Material prices are our own ranges, set from retail and roofing-supplier list prices (last checked October 2026). The low end uses every low assumption together and the high end every high one, so real quotes usually fall between them.
| 3-tab asphalt shingles, per square | $90 / $105 / $126 |
|---|---|
| Architectural asphalt shingles, per square | $114 / $135 / $174 |
| Designer asphalt shingles, per square | $200 / $280 / $400 |
| Accessories (underlayment, starter, cap, drip edge, nails, vents), per square | $25 / $40 / $60 |
| Material markup | 1.1× / 1.15× / 1.25× |
| Install labor, crew hours per square | 1.4 h / 1.8 h / 2.3 h |
| Tear-off labor, crew hours per square per layer | 0.5 h / 0.7 h / 1 h |
| Wage-to-billed-rate multiplier | 2.5× / 3.2× / 4× |
| Disposal, per square per layer | $10 / $18 / $30 |
| Decking sheet (4 × 8 ft), material | $18 / $24 / $32 |
| Permit | $100 / $250 / $500 |
| System | Panels, per square | Trim and accessories, per square | Install, crew hours per square | Min. pitch | Assumed life |
|---|---|---|---|---|---|
| Exposed-fastener steel panels | $130 / $180 / $250 | $40 / $60 / $90 | 1.5 h / 2 h / 2.6 h | 3/12 | 35 yr |
| Standing-seam steel | $380 / $520 / $700 | $80 / $120 / $170 | 3 h / 4 h / 5.5 h | 0.25/12 | 50 yr |
| Stone-coated steel | $400 / $520 / $650 | $70 / $100 / $140 | 2.5 h / 3.2 h / 4.2 h | 3/12 | 45 yr |
| Standing-seam aluminum | $550 / $750 / $1000 | $90 / $130 / $180 | 3 h / 4 h / 5.5 h | 0.25/12 | 50 yr |
Installing metal over one layer of shingles adds purlins at $15 / $25 / $40per square plus 0.3 h / 0.4 h / 0.6 h per square, and skips tear-off. For cost-per-year comparisons we assume architectural shingles last 25 years. These service lives are planning figures, not warranties.
| Central AC with coil: base + per ton | $1200 / $1600 / $2200 + $450 / $600 / $850 |
|---|---|
| Heat pump with air handler: base + per ton | $1500 / $2000 / $2800 + $550 / $750 / $1050 |
| Gas furnace (96% AFUE): base + per 20,000 BTU/h input | $1100 / $1500 / $2200 + $80 / $120 / $170 |
| Mini-split, per indoor unit | $1200 / $1700 / $2500 |
| Efficiency multiplier (standard / high / top) | 1× / 1.3× / 1.7× (furnaces 0.8× / 1× / 1.35×) |
| Equipment markup | 1.2× / 1.35× / 1.5× |
| Crew hours, ac | 8 h / 11 h / 16 h |
| Crew hours, heat pump | 10 h / 13 h / 18 h |
| Crew hours, furnace | 6 h / 9 h / 14 h |
| Crew hours, ac furnace | 14 h / 19 h / 26 h |
| HVAC wage-to-billed-rate multiplier | 2.8× / 3.6× / 4.5× |
Electrical assumptions
- Electricity costs use the EIA's year-to-date average residential price for each state unless you enter your own rate.
- Voltage drop uses the AWG diameter formula and conductor resistivity at 75 °C: 12.9 ohm·cmil/ft for copper and 21.2 for aluminum. It never sizes wire for ampacity.
- Generator sizing uses typical running watts and a starting surge of about 3× running for motor loads, adds the largest single surge, then 10% headroom, and keeps running load at or below 80% of the rating.
- EV charging adds 10% for charging losses. Level 1 is 120 V at 12 A; Level 2 is 240 V at 32 or 48 A.
Calculators that use these methods
Roofing
- Attic Ventilation Calculator
- Gutter Size Calculator
- Metal Roof Cost
- Roof Pitch Calculator
- Roof Replacement Cost
- Roof Square Footage Calculator
- Shingle Calculator
- Underlayment Calculator
HVAC
- AC Size Calculator
- Duct Size Calculator
- Furnace Size Calculator
- Heat Pump Size Calculator
- HVAC Replacement Cost
- SEER Savings Calculator
Electrical
- Electricity Cost Calculator
- EV Charging Cost Calculator
- Generator Size Calculator
- Voltage Drop Calculator
What we don't use
We don't copy figures from commercial cost databases or other calculator sites. Our numbers come from government data, published standards and our own calculations.
Updates and corrections
We refresh the dataset and cost ranges every January and whenever a source publishes a major update. Each page shows its own "Last updated" date. Found an error? Email info@homeyardstick.com.