Concrete CO₂ / Carbon Footprint Calculator

Frequently Asked Questions

• How much CO2 does concrete produce per cubic yard? +
  A typical 3,500 PSI ready-mix concrete with 100% Portland cement produces approximately 194 kg CO₂e per cubic yard (roughly 254 kg CO₂e per m³). With 25% fly ash substitution, this drops to around 146 kg CO₂e per cubic yard. The exact figure depends on your cement content per yard, the clinker factor of the cement, and any supplementary cementitious materials used. Use this calculator with your actual mix specs for a project-specific answer rather than relying on an industry average.
• What is embodied carbon in concrete and why does it matter? +
  Embodied carbon refers to the greenhouse gas emissions generated during the manufacture, transport, and installation of a building material — in this case concrete — as opposed to operational carbon (the energy used to heat or cool the finished building). Embodied carbon in concrete is essentially locked in at the time of the pour and cannot be reduced after the fact. As buildings become more energy-efficient and operational carbon falls, embodied carbon represents an ever-larger share of a structure's total lifetime carbon footprint. For some highly efficient buildings, embodied carbon now accounts for 50–80% of lifetime emissions.
• How does fly ash reduce concrete carbon emissions? +
  Fly ash is a by-product of coal combustion at power stations. It is a pozzolan — when mixed with water and Portland cement, it reacts with calcium hydroxide to form additional calcium silicate hydrate (CSH), the same binding compound responsible for concrete's strength. Because fly ash is a waste product with a very low allocated carbon footprint (~0.004 kg CO₂e/kg fly ash), replacing a portion of Portland cement (which has a high carbon intensity of ~0.820 kg CO₂e/kg) with fly ash reduces the overall embodied carbon of the mix proportionally. A 25% fly ash replacement typically reduces embodied carbon by 22–25% with no structural downside at typical residential and commercial strengths.
• What is the difference between fly ash and GGBS (slag) in concrete? +
  Both fly ash and Ground Granulated Blast-furnace Slag (GGBS, also called slag cement) are supplementary cementitious materials that reduce embodied carbon by replacing Portland cement. Fly ash is a coal combustion by-product; GGBS is a steel manufacturing by-product. GGBS is generally considered a more reactive SCM — it achieves comparable final strength to Portland cement at higher replacement rates (up to 70% in some applications) but has even slower early strength gain than fly ash. GGBS typically has a carbon footprint of around 0.052–0.083 kg CO₂e/kg — still much lower than Portland cement at 0.820 kg CO₂e/kg. GGBS is more common in the UK and Europe; fly ash is the dominant SCM in the US.
• Does high-fly-ash concrete take longer to cure? +
  Yes. High-volume fly ash mixes (typically 30–40%+ replacement) develop strength more slowly than straight Portland cement mixes. At 7 days, a 40% fly ash mix may have only 60–70% of its 28-day strength, compared to 70–80% for a standard mix. By 28 days, most fly ash mixes reach full design strength. By 56 or 90 days, they often exceed the strength of comparable Portland-only mixes because the pozzolanic reaction continues. The practical implication: do not load fly ash slabs heavily or strip forms early. If you have a tight construction schedule that requires early stripping, either use less SCM, add an accelerator, or account for the slower gain in your schedule. At 25% fly ash substitution, the difference in early strength is modest and rarely a scheduling issue.
• How does concrete carbon compare to other building materials? +
  On a per-kg basis, concrete (roughly 0.08–0.15 kg CO₂e/kg) has a lower carbon intensity than steel (1.5–2.5 kg CO₂e/kg for primary steel) or aluminium (8–12 kg CO₂e/kg). However, concrete is used in vastly greater quantities — the world produces more than 30 billion tonnes of concrete per year (about 14 billion m³, made with roughly 4 billion tonnes of cement), making the cement industry one of the largest single sources of CO₂ globally. Structural timber (mass timber products like CLT) stores carbon rather than emitting it during growth and is often cited as an alternative, though it cannot fully replace concrete structurally in all applications. The most carbon-efficient strategy for most projects is to optimise concrete volume through good structural design, and then reduce the cement content per cubic meter through SCMs — rather than switching materials entirely.
• What is an EPD and do I need one for my project? +
  An Environmental Product Declaration (EPD) is a standardised, third-party verified document that discloses the environmental impacts — including embodied carbon — of a specific product. For concrete, EPDs are issued by the ready-mix plant or concrete manufacturer for their specific mix designs. If your project targets LEED v4/v4.1 Materials & Resources credits, a Buy Clean procurement program that covers concrete (such as Buy Clean Colorado or New York's Buy Clean Concrete guidelines), or any green building certification that requires documented embodied carbon, you will need plant-specific EPDs from your concrete supplier. This calculator uses representative industry emission factors for estimation purposes; for formal reporting and compliance, always use your supplier's EPD data.
• Can concrete absorb CO2 over its lifetime (carbonation)? +
  Yes. Concrete undergoes a process called carbonation in which atmospheric CO₂ reacts with calcium hydroxide (portlandite) in the hardened cement paste to form calcium carbonate. Over a structure's service life, concrete can reabsorb a portion of the CO₂ emitted during cement production — estimates range from 5% to 30% of production emissions over 50–100 years, with the higher end applying to demolished and crushed concrete which exposes much more surface area. This carbonation uptake is real but not included in standard embodied carbon calculations (per EN 15978) or LEED accounting, because it occurs over a long timeframe and is secondary to the upfront emission. It is a legitimate factor in full life-cycle analyses of concrete infrastructure.
• Does recycled concrete aggregate (RCA) reduce carbon emissions? +
  Recycled concrete aggregate (RCA) from demolished structures can replace a portion of virgin coarse aggregate in new concrete mixes. Because aggregate (sand and gravel) has a very low embodied carbon per kilogram (~0.005–0.010 kg CO₂e/kg), substituting RCA for virgin aggregate saves only a small amount of carbon directly. The larger benefit is avoiding landfill disposal of demolition waste and reducing quarrying impacts. RCA is generally not a major lever for carbon reduction in concrete — SCMs (fly ash, GGBS) are far more impactful because they reduce the high-carbon cement fraction.
• How accurate are this calculator's emission estimates? +
  This calculator uses representative emission factors based on IPCC AR6 Working Group III data and published industry EPDs for typical US ready-mix concrete. For most estimation and comparison purposes, the results are accurate within 10–20%. However, actual emissions vary depending on: the specific clinker-to-cement ratio of your supplier's cement; the exact SCM type and source (not all fly ashes or slags are identical); regional energy grid carbon intensity affecting plant operations; and the actual batch weights rather than nominal PSI-based estimates. For LEED documentation, green building certification, or formal carbon reporting, obtain plant-specific EPDs from your ready-mix supplier and use the measured values.
• How many pounds of CO2 are in a yard of concrete? +
  With this calculator's default factors, one cubic yard of 3,500 PSI concrete made with 100% Portland cement carries about 428 lb of CO₂e (194 kg). The same yard with 25% fly ash is about 322 lb (146 kg), and a 4,000 PSI yard with no SCM is about 498 lb (226 kg). These figures cover the cement and SCM only. A 15-mile haul from the plant adds roughly 12 lb per yard. The results panel shows both kg CO₂e per yd³ and the total in pounds for your own inputs.
• Should I enter my own cement content or just pick a PSI? +
  Enter your own figure whenever you have it. Strength alone does not fix how much binder a mix contains, and two 4,000 PSI mixes from different plants can carry different amounts. Your batch ticket or mix design submittal lists the cementitious content in lb/yd³ or kg/m³. If the mix is described in sacks, multiply by 94 lb: a 6-sack mix is 564 lb/yd³, or about 335 kg/m³. Enter the total of cement plus fly ash or slag in the Cementitious Content field, then choose the SCM percentage separately. If you only know the strength, leave the field blank and the calculator uses the typical value shown in the assumptions table below.
• Does portland-limestone cement (Type IL) lower the carbon footprint of concrete? +
  Yes. Type IL cement is made by intergrinding up to 15% limestone with the clinker, so each tonne contains less clinker. The Portland Cement Association's 2021 industry-wide EPDs put US portland-limestone cement at 846 kg CO₂e per tonne, against 922 kg CO₂e per tonne for ordinary portland cement, which is about 8% lower. This calculator applies a single factor of 0.820 kg CO₂e per kg to the Portland cement portion of the mix, which already sits at the Type IL end of that range, so do not subtract a further Type IL allowance from the result. If your supplier's cement EPD shows a different figure, scale the cement production line of your result in proportion.
• Does this calculator include rebar, aggregates, or pumping? +
  No. The result covers the cement and SCM in the mix and, if you enter a distance, the truck haul from the plant to the site. Aggregates, water, admixtures, batching plant energy, reinforcing steel, formwork, pumping and placing are not counted. Reinforcement can be a significant share on heavily reinforced work, so estimate the steel tonnage with the Rebar Calculator and account for it separately using the steel supplier's EPD.