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Fly Ash / Silica Fume SCM Replacement Calculator

Split a concrete mix's cementitious material into portland cement and a supplementary cementitious material (SCM) by mass, at a target replacement percentage.

Computed in your browser — nothing you enter is uploaded. Figures are presented in United States units and terminology, and each formula’s basis is cited under regulatory standards below.

Last verified 2026-08-27 · v1.1.0

Market
Imperial · sales tax

Portland cement required

17.44 pcf

High confidence
SCM mass required
4.36 pcf

Running these inputs gives 17.4 pcf as the portland cement required. Scm mass required carries the most weight in this calculation, at 4.36 pcf. Currently reading for United States — pick a different market above and the figures re-cast accordingly.

Add the equipment this sizes

This result is a specification — 17.44 pcf — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.

Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Regulatory standards & verification citations

  • Supplementary cementitious materials commonly replace 15-30% of portland cement by mass for fly ash (ACI 232.2R) and 5-10% for silica fume (ACI 234R), reducing embodied carbon while can improving long-term strength and durability

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Frequently asked questions

Why replace portland cement with an SCM?
Portland cement production is a major source of embodied carbon; fly ash and silica fume are industrial byproducts that reduce cement demand while typically improving long-term strength, reducing permeability, and often improving workability.
Does using an SCM slow down strength gain?
Usually yes for fly ash, especially at early ages (the first 7-28 days) — the mix design and curing schedule should account for this, particularly on fast-track construction schedules.
Can I combine fly ash and silica fume in the same mix?
Yes, ternary blends (portland cement + fly ash + silica fume) are common in high-performance concrete — this calculator handles one SCM type at a time; run it twice and subtract for a ternary blend.