Comparison

Best free concrete calculators for ACI 318 design (2026)

This is a side-by-side comparison of every free concrete calculator we could find that performs structural capacity checks to ACI 318. Volume and takeoff estimators are excluded; this list covers tools that compute interaction diagrams, check slenderness, or verify member capacity.

calcnote is one of the tools compared. We include it on the same terms as the others and call out where competitors do things calcnote does not. concreteproperties (a Python library) is included for completeness even though it does not natively support ACI 318; it targets AS 3600 and Eurocode 2.

Quick comparison

Dimension	calcnote	calcs.app	CivilStrCalc	ISS/civilcalc	concreteproperties	Eagle Eye (free)	SkyCiv (free)
Price	Free, no login	Free	Free	Free	Free (Python)	Free (5 projects)	Free (basic)
ACI edition	318-19 and 318-25	~318	318-25	Unclear	No ACI	318-19	318-19
P-M method	Exact integration	Not confirmed	Not confirmed	Basic	Exact (code-only)	Not confirmed	~40 points
Biaxial bending	Exact 3D (inclined NA)	P-M2-M3	Bresler	No	Exact (code-only)	3D P-M-M	Rotated analysis
Slenderness	Sway + non-sway	No	No	No	No	Sway + non-sway	Limited
Circular columns	Yes	Sectional	Tied + spiral	No	Arbitrary	Tied + spiral	Yes
Beams	Flexure, shear, T-beam, deflection, crack	Not confirmed	Not confirmed	Flexure only	No	Not confirmed	Basic (gated)
Footings	Isolated + wall	No	No	No	No	Not confirmed	Gated
Reports	PDF + .docx	Print	Steps	Minimal	Plots (code)	Gated	Gated
Login required	No	No	No	No	N/A	Yes	Yes

"Not confirmed" means we could not verify the capability from public documentation.

Tool-by-tool breakdown

calcnote

calcnote uses exact strain-compatibility integration per ACI 318-19 and ACI 318-25 Section 22.2, the same method as spColumn. It solves for the inclined neutral axis directly for biaxial bending, rather than using the Bresler reciprocal approximation. Sway and non-sway slenderness checks use moment magnification per ACI 318-19 and ACI 318-25 Sections 6.6.4 and 6.6.4.6.

Beyond columns, calcnote covers beams (flexure, shear, T-beam, Bischoff I_e deflection, crack control per Cl 24.3.2), footings (isolated + wall, with punching shear, interface shear, and dowel checks), and ASCE 7-22 load combinations. All in one free tool with no login, no project caps, and PDF + .docx export.

The engine is cross-validated against spColumn within 0.03% on nominal capacity across 51 test cases. See the methodology hub for how calcnote implements ACI 318-19 and ACI 318-25, or try the sample calculations.

Best for: engineers who need a full ACI 318-19 or ACI 318-25 design suite (columns + beams + footings) at zero cost with no friction.

CivilStrCalc

CivilStrCalc targets ACI 318-25, the newest edition of the code. calcnote also supports 318-25 (selectable per calculation, with 318-19 as the default), while most other free tools are still on 318-19. It uses the Bresler reciprocal approximation for biaxial bending and supports tied and spiral column configurations.

It does not include slenderness analysis, beam design, or footing checks. Output is step-by-step text rather than a downloadable report.

Best for: engineers who need ACI 318-25 compliance for column-only checks.

calcs.app

calcs.app is a free web-based tool with P-M2-M3 interaction analysis and sectional circular column support. Public documentation is limited, making it difficult to verify the exact ACI edition or analysis method used.

It does not include slenderness analysis or footing design. Output is a browser print sheet.

Best for: quick column-only P-M checks.

ISS / civilcalc

ISS/civilcalc is the most basic tool in this comparison. It handles uniaxial flexure only (no biaxial bending, no slenderness), with an unclear ACI edition reference. It offers flexure-only beam checks but no footing design.

Best for: very rough preliminary flexure checks.

concreteproperties (Python library)

concreteproperties is an open-source MIT-licensed Python library for analyzing reinforced concrete cross-sections of arbitrary geometry. It computes moment-curvature relationships, interaction diagrams, and biaxial bending diagrams using exact strain-compatibility integration.

It targets AS 3600 and Eurocode 2 (no ACI 318 module). It has no web interface: you write Python scripts to define sections and run analyses. It does not perform member-level checks (slenderness, shear, deflection).

Best for: researchers, developers, non-ACI code checks, and irregular cross-sections that other free tools cannot handle.

Eagle Eye (free tier)

Eagle Eye offers ACI 318-19 column design with sway and non-sway slenderness and 3D P-M-M interaction on its free tier. The free tier is capped at 5 projects, and .docx report export requires the $39/mo paid plan.

It supports tied and spiral circular columns. Beam and footing capabilities could not be confirmed from public documentation.

Best for: engineers who need a few quick checks and are OK with the 5-project limit.

SkyCiv (free tier)

SkyCiv's free calculator provides a basic ACI 318-19 column check using approximately 40 points on the P-M curve. The full platform ($109-179/mo) adds slenderness, multi-code support (ACI + 4 other codes), footings, and detailed reports.

The free tier is heavily feature-gated: slenderness, footings, and branded PDF reports all require a subscription.

Best for: engineers already on the SkyCiv platform who want a quick free check.

What makes a free calculator trustworthy?

A free price tag does not mean low quality, but it does mean you should check the tool's verification before using it on a real project. Look for:

Published test cases: does the tool show cross-validation against a known reference (textbook examples, established software)?
Step-by-step output: can you trace the calculation path, or is the result a black box?
Code compliance documentation: does the tool cite specific ACI provisions for each check?

calcnote publishes 51 test cases cross-validated against spColumn and Wight textbook examples, with full witness pages showing the comparison data.

Frequently asked questions

Which free concrete calculator is the most accurate?

Among free web-based ACI 318 tools, calcnote uses exact strain-compatibility integration per ACI 318-19 and ACI 318-25, the same method as spColumn ($2,295). Its engine is cross-validated against spColumn within 0.03%. Other free web-based ACI tools use approximate methods (Bresler) or have unverified accuracy. The open-source Python library concreteproperties also uses exact integration but targets AS 3600 / Eurocode 2 (no ACI 318 module) and requires scripting.

Are free concrete calculators reliable for real projects?

It depends on the tool's verification depth. calcnote publishes 51 test cases cross-validated against spColumn and Wight textbook examples. Check any tool's documentation for published verification before relying on it for submittals.

Do any free tools support ACI 318-25?

CivilStrCalc targets ACI 318-25. calcnote supports both ACI 318-19 and ACI 318-25 (selectable per calculation), while most other free tools support only ACI 318-19. For rectangular and circular column strength design, the core provisions in Chapter 22 are substantially unchanged between 318-19 and 318-25. Other member types may see larger differences (e.g., revised shear friction provisions).

Can I use a free calculator instead of spColumn or ENERCALC?

For rectangular and circular columns with standard bar layouts, calcnote covers the same analysis scope as spColumn (exact integration, sway + non-sway slenderness, 3D biaxial) and also includes beams, footings, and load combinations. For irregular sections or non-ACI codes, paid tools remain necessary. See the calcnote vs spColumn comparison for a detailed head-to-head.

Is there a free tool that handles columns, beams, and footings together?

calcnote is the only free tool that covers columns, beams, footings, and load combinations in a single platform. Other free tools focus on columns only or on a single member type.