We run a free Manual J-style load calculator. Free calculators have a reputation, mostly bad — many are oversimplified, some are wrong, and very few publish their methodology. This page exists because we think calculators that affect equipment sizing decisions worth thousands of dollars should be auditable, and we want to make that auditing easy.
Below we cover what the calculator implements (Manual J 8th Edition methodology, ASHRAE 169 climate data, documented infiltration and solar gain models), the three-layer verification framework it is designed against (ACCA reference cases, peer-software comparison, real-home utility data), the public audit surface that lets any reader check our work, and what the calculator does not yet handle. We do not claim accuracy results we have not run.
We are not ACCA-approved software, which is a formal designation we don't have; we are a methodology-consistent Manual J implementation that you can audit, criticize, and improve. For broader context, our methodology covers the approach across all calculators on the site.
What Manual J Is (Brief)
Manual J is the residential heating and cooling load calculation standard published by ACCA (Air Conditioning Contractors of America).[1] See ACCA Manual J 8th Edition for the source document. It calculates the design heating load (BTU/hr at the 99% heating design temperature) and the design cooling load (BTU/hr at the 1% cooling design temperature) for a residential building based on envelope characteristics, climate, internal gains, and occupancy.
For the full treatment, see the Manual J methodology overview article. The Manual J-style load calculator implements this methodology with simpler inputs than commercial software, designed for planning rather than permit submission.
We follow Manual J 8th Edition (ACCA, 2016). This is the current published version as of 2026. Methodology updates and addenda are incorporated as ACCA publishes them.
Why Verification Matters
A wrong load calculation propagates downstream: equipment selection wrong, ductwork sized wrong, Manual S equipment selection defective, performance suffers for the life of the system. The cost of a wrong load is not the calculator's $0 price tag; it is the $5,000-$15,000 equipment purchase and 15-20 years of operating cost based on that purchase.
Three failure modes if a calculator is systematically wrong:
- Oversize systematically. Results in short cycling, humidity control problems, accelerated equipment wear, and wasted capital
- Undersize systematically. Results in comfort failures on design days, heavy auxiliary heat use in cold climates, and undersized cooling on the worst summer days
- Methodology errors that work in normal cases but fail at edge cases. Hardest to spot; surface only under specific conditions
Field-installed HVAC has well-documented sizing errors of 30-50% in many cases (DOE field studies and HVAC efficiency research consistently show oversizing as the dominant pattern). A free calculator that's wrong perpetuates the problem. A free calculator that publishes its methodology and audit channel can help fix it. Heat pump sizing context and home heat loss methodology cover the downstream consequences of getting Manual J wrong.
This page documents the methodology, the verification framework, and the audit surface so any reader can check the work.
The Methodology the Calculator Implements
For the technically curious or auditors reviewing the implementation:
Inputs and how they're used. Building envelope (areas, R-values, U-factors), orientation, climate zone (mapped to nearest ASHRAE Standard 169 location), infiltration assumption (default or user-entered blower-door data), occupancy and internal gains schedule.[4]
Climate data. ASHRAE 169 design temperatures, 1% cooling and 99% heating. Embedded as a static lookup table for ~5,000 US locations. Updated annually as ASHRAE 169 updates.
Solar gain calculations. ASHRAE-recommended solar heat gain factors by orientation (N, NE, E, SE, S, SW, W, NW) and latitude.[3] Glazing types: clear single, clear double, Low-E double, Low-E triple. Different SHGC values per glazing type.
Infiltration. Blended Sherman-Grimsrud / ASHRAE enhanced model. Default infiltration rates by home age (matches ResStock defaults); user can override with blower-door ACH50 data.
Internal gains. ANSI/RESNET/ICC 301 default occupancy schedule. Standard occupancy assumed; non-standard high-density use (home offices, gyms) requires manual adjustment via the calculator's advanced settings.
Duct gains and losses. Manual J Appendix-D approximation for ducts in unconditioned space. Detailed Manual D handling not yet automated.
Software implementation. Pure-JavaScript front-end calculation, deterministic (same inputs produce same outputs every time), no random factors, no opaque transformations. Each calculation step uses documented formulas with version references in the source code.
The Verification Framework (Three Layers)
A Manual J implementation can be checked against three independent layers. The calculator is designed against all three.
Layer 1: ACCA published reference cases. ACCA publishes the Manual J 8th Edition Approval Test Suite, a set of 8 reference homes with documented inputs and expected output loads.[2] Software vendors seeking ACCA approval run these cases as part of the formal certification process. The cases cover different house types, climate zones, and construction styles (1-story ranch zone 3, 2-story colonial zone 4, split level zone 5, small bungalow zone 2, large modern zone 6, passive solar zone 4, manufactured home zone 7, walkout basement zone 5).
Verification target band: ±5% on each case. This is the strongest verification baseline because ACCA defines both the inputs and the expected outputs, so there is no input ambiguity.
Layer 2: Peer-implementation comparison. A Manual J implementation can be cross-checked against established ACCA-approved software (Wrightsoft Right-J, Cool Calc Manual J, reference Manual J 8 spreadsheet implementations) on identical inputs. Discrepancies that surface trace to allowed-by-Manual-J assumption differences (default infiltration when no blower-door data is provided, internal gains schedule selection, solar gain factor interpretations at edge orientations) rather than calculation errors. The Manual J comparison literature documents inter-implementation variance on identical inputs at typically ±3-5%, which is the verification target band for this layer.
Layer 3: Real-home validation. The calculator's output can be compared to actual peak demand inferred from real-home utility data (with baseline non-HVAC load subtracted) and from datasets like NREL ResStock[6] and the BuildingSync standardized energy audit schema.[5] This is the hardest layer because it combines calculator accuracy with the inherent uncertainty in real-home input data and the difficulty of measuring true peak HVAC demand from utility data. The target band is ±15% on most homes, consistent with Manual J's inherent input-uncertainty floor documented in the building-science literature. Outliers beyond ±15% typically trace to user input errors (square footage misreporting, insulation assumptions that don't match the actual home) rather than calculator errors.
These three layers are what an honest verification looks like for a Manual J implementation. They are the framework the calculator is designed against. Any reader running the calculator against any layer can submit results through the audit channel; verified discrepancies that violate the methodology are bugs and get logged in the corrections record.
The Audit Surface
What is publicly available for audit:
- The methodology documentation. This page describes what the calculator implements and the verification framework it targets. Anyone can check whether the implementation matches the stated methodology by reading the source code alongside this page.
- The formulas and source code. Plain-language formula references in the source code repository, no obfuscation. Each calculation step uses documented formulas with version references.
- The known-limitations list. Updated openly when discovered. Honest about where the calculator is not strong.
- The corrections log. Material updates triggered by reader-reported errors or methodology disagreements are documented at /corrections/ with date, original claim, corrected claim, and (with reporter consent) attribution.
- The audit channel. Any reader can run a verification at any of the three layers above and report results through the contact channel.
The audit surface is the alternative to unverified marketing claims. Any reader can check the implementation against the stated methodology at any time; documented discrepancies surface as bug reports; fixed bugs get logged in the corrections record.
What We Don't Claim
The calculator is not perfect, and we don't pretend it is. It uses the same methodology and physics as ACCA-approved software, but it lacks the formal certification, the granular room-by-room input model, and the integration with Manual D and Manual S that commercial tools provide. It approximates non-rectangular geometries. It assumes standard occupancy schedules.
It estimates duct losses rather than measuring them. These are real limitations and they affect real users. If your situation matches one of them, use ACCA-approved software or a certified contractor instead. If it doesn't, this is a useful tool for planning and evaluation. But check the results against your own intuition and consult a professional before installing equipment.
Honest limitations published openly:
- Non-standard building geometries. Round houses, A-frames, earth-bermed homes use simplified rectangular approximations. Expect ±15-20% accuracy in those cases
- Multi-zone diversity. The calculator produces whole-house loads. Per-zone analysis requires individual Manual J runs per zone, which the tool does not yet automate
- Internal load schedules. Standard occupancy assumed. Home offices, gyms, server rooms, dense workshops require manual adjustment beyond the calculator's default inputs
- Ductwork in unconditioned space. Duct loss factors estimated, not measured. Detailed Manual D is required for accurate duct loss in tight or high-performance designs
- Older homes with unknown insulation. Outputs depend entirely on user inputs. Garbage in, garbage out
We are explicitly NOT ACCA-approved software. That's a formal designation involving certification fees and process; we don't have it. We are a planning-grade tool.
When in doubt, use ACCA-approved software or a certified contractor. The ACCA-compliant Manual J path runs through certified vendors, not free web calculators.
How to Audit Us
Any reader can audit the calculator at any of the layers in the verification framework. Manual J audit can mean comparing the calculator against your own Manual J inputs and expected outputs, against another ACCA-approved tool, against published reference cases, or against your own home's measured energy data.
How to verify HVAC load calculation against this tool:
- Run the calculator with your specific inputs
- Compare to your expected output against your own Manual J calculation, another calculator, or rule-of-thumb sanity check
- Email the audit channel with discrepancies you can't explain
- Review the methodology section above to check the implementation against documented assumptions
- Check the corrections log at /corrections/ for recent changes that might affect your case
What we commit to:
- Responding to documented audit reports within a few business days
- Fixing verified bugs and logging the fix in the corrections record
- Discussing methodology disagreements openly (different defaults aren't bugs; the choice is explained against the cited standard)
- Crediting reporters who improve the tool (unless they prefer anonymity)
The audit contact runs through the contact page. Include your inputs as entered, the calculator's output, what you expected and why, and any third-party calculator outputs you ran for comparison.
What counts as a verified bug: a documented case where the calculator produces a result that violates Manual J 8th Edition methodology, ASHRAE physics, or its own published reference case behavior. What counts as a methodology disagreement rather than a bug: a different infiltration default than another calculator (both defensible under Manual J), a different solar gain factor table for the same orientation (ASHRAE publishes multiple acceptable tables), or differences in rounded final results.
This is the alternative to unverified claims. Published methodology, bug reporting, visible accountability, and an open audit channel are the trust mechanisms a free calculator can offer; "trust us" is not.