Enhanced Rock Weathering CO₂ Removal Calculator

Introduction: How enhanced rock weathering removes CO₂

Enhanced rock weathering (ERW) uses finely ground silicate rock as a carbon removal feedstock. When that material is spread on soil or another exposed surface, calcium- and magnesium-bearing minerals dissolve more quickly than they would inside intact rock. The dissolved ions can then be carried away as bicarbonate in water or locked into carbonate minerals, which is why ERW is often discussed as a way to couple farm management with carbon removal. The estimate depends on the chemistry of the rock, the amount you spread, and how much of the material actually reacts after application.

This calculator focuses on the CaO and MgO portions of the rock because those oxides are the parts that most directly translate into potential CO₂ uptake in this simplified model. The calculator applies the same stoichiometric idea used in ERW accounting: a kilogram of calcium oxide can correspond to a little under four-fifths of a kilogram of CO₂ removed, and a kilogram of magnesium oxide can correspond to a little more than one kilogram of CO₂ removed if the material reacts fully. Real feedstock is a mixture, so the applied mass and oxide percentages determine how much reactive mineral is available. Rainfall, grain size, soil biology, mixing depth, and incorporation method then shape how much of that potential is realized over the project period. The result is therefore a planning estimate for comparing feedstocks and application rates, not a guaranteed field outcome.

The equation behind the calculator is intentionally compact. It starts with the total rock mass, scales that by the CaO and MgO assay values, converts each oxide into a carbon-dioxide equivalent using the stoichiometric factors built into the form, and then applies the fraction expected to react. In other words, the tool estimates the chemistry-side ceiling for removal, then reduces that ceiling to match your assumption about how much of the amendment will actually weather. That makes it useful for early-stage planning, sensitivity checks, and comparing one quarry source against another without pretending to replace field monitoring.

Worked example: Basalt, olivine sand, and dunite in enhanced rock weathering

Rock choice matters because the oxide mix sets the upper bound on potential CO₂ removal. Basalt usually carries only modest amounts of CaO and MgO, while olivine-rich sands and ultramafic rocks can contain much more magnesium. The table below compares three common materials on a per-tonne basis so you can see why the same application rate can lead to different removal estimates. The values assume complete reaction and therefore represent chemistry-only ceilings rather than field outcomes, which is exactly the perspective a quick calculator is meant to provide before a project team spends time on transport, spreading, and site preparation.

In practice, the table should be read as a ranking tool. Higher MgO content generally pushes the estimate upward, but that advantage can be offset if the material is hard to grind, expensive to transport, or slow to dissolve in local conditions. Project teams often use a comparison like this when deciding whether a locally available basalt is good enough or whether a higher-grade mineral source is worth the extra processing burden. The calculator then lets you test the same logic with your own rock assay and reaction assumption instead of relying on generic field averages.

Applying the Calculator: Turning rock assays into an ERW estimate

To use the enhanced rock weathering calculator, start with the amount of crushed rock you plan to apply and enter it in tonnes. Then enter the CaO and MgO assay values from a lab report, supplier sheet, or published rock description. The reaction fraction should reflect how much of those oxides you think will actually dissolve during the period you care about, whether that is a season, a year, or a longer project horizon. Because the calculator is meant for planning, it is more useful to choose a defensible assumption than to chase false precision.

Once you submit the numbers, the browser multiplies the rock mass by each oxide share, applies the carbonate conversion factors, and scales the result by the reacted fraction. That gives a quick estimate of CO₂ removal in tonnes. Because the calculation happens instantly, you can test alternate feedstocks, compare low- and high-reaction cases, or see how sensitive the result is to a small change in assay values before making a sourcing decision. That is especially helpful when you are comparing a nearby quarry with a stronger but more distant mineral source.

Environmental and Agronomic Considerations for Enhanced Rock Weathering

ERW is often discussed as a carbon removal method, but in the field it behaves like a mineral amendment first and a climate intervention second. As the rock dissolves, it can influence soil pH and add nutrients such as calcium, magnesium, potassium, or trace elements depending on the source rock. Those side effects may improve crop performance, but they also mean the right material in one field may not be the right material somewhere else. A calculator can estimate the carbon side of the trade-off, yet it cannot judge whether the agronomic effects suit your crop, soil, irrigation plan, or management goals.

Weathering speed still depends on familiar field conditions: moisture, temperature, biological activity, mixing depth, and particle size. Finer grinding gives more surface area and usually faster dissolution, though the extra energy required to make that powder affects the project’s overall carbon balance. Runoff, erosion, and incomplete incorporation can all leave material unreacted, so the estimate should be treated as a planning ceiling until field data confirm how the rock behaves on site. Those practical limits are why the calculator asks you for a reaction fraction instead of pretending every tonne of rock will behave the same way.

Uncertainties and Research Frontiers in Enhanced Rock Weathering

The main uncertainty in ERW accounting is not the chemistry itself; it is the speed and completeness of that chemistry in messy real-world conditions. Laboratory assays can tell you the CaO and MgO content with confidence, but translating those numbers into long-term CO₂ removal requires assumptions about climate, soil chemistry, particle size, and how often the material is disturbed or washed away. Life-cycle emissions from mining, grinding, and transport also matter, because a rock source that looks strong on paper may deliver much less net removal once the whole supply chain is included. A simple calculator cannot solve those uncertainties, but it can make them visible by showing how strongly the result changes when the inputs change.

Researchers are actively testing ways to tighten those estimates. Better monitoring of runoff chemistry, isotopic tracing, and field lysimeters can help show how much carbon actually leaves the soil system. Some projects also explore low-carbon grinding, renewable electricity, or alternative feedstocks such as steel slag to improve the net balance. As the science matures, calculators like this one remain useful for early-stage comparisons, but they work best when paired with site measurements and a project-specific carbon accounting method. That is especially true for projects that hope to sell credits or report verified carbon removal.

Exploration Through Numbers: Comparing ERW scenarios before you commit

Scenario testing is one of the most useful ways to work with an enhanced rock weathering calculator. You can compare a local basalt source against a magnesium-rich ultramafic source, ask how much the estimate drops if only part of the amendment reacts, or see whether a modest improvement in oxide content is worth the additional hauling distance. The point is not to memorize a single answer; it is to understand which assumption has the biggest influence on the estimate so you know where to focus your due diligence.

This is also where the calculator earns its keep. You can compare a locally abundant rock against a higher-reactivity import, test the effect of a lower reaction fraction on dry sites, or see whether a small improvement in CaO or MgO content changes the project enough to matter. The result is not a promise of verified removal; it is a way to organize choices before money, labor, and land are committed. If one assumption drives the estimate much more than the others, that is a clue to double-check the assay, the spreading plan, or the field conditions that govern weathering.

Beyond farm fields, enhanced rock weathering has been studied for beaches, tidal zones, and restoration sites where mineral buffering may help manage acidity while capturing CO₂. Those settings still require location-specific calculations because water movement, sediment turnover, and application method can all change how quickly the rock reacts. Even so, the same chemistry applies: more reactive mineral, more contact with water, and more time to weather generally mean a larger estimate. The calculator can therefore serve as a quick screening step for a variety of ERW concepts, even when the final application setting differs.

Enhanced rock weathering is not a silver bullet, but in combination with emissions reductions and other carbon removal strategies, it can become a meaningful part of a broader climate plan. Transparent assumptions make it easier for farmers, carbon project developers, policymakers, and researchers to compare options without overstating what a single amendment can do. By testing your own inputs, you get a clearer sense of where the chemistry is strong, where the uncertainty is high, and where field validation still matters. That makes the calculator useful not only for project design, but also for conversations with suppliers, agronomists, and auditors.

How to use this enhanced rock weathering calculator

1. Enter Rock Mass Applied (tonnes) as the total amount of crushed material you plan to spread; the estimate scales directly with that mass.
2. Enter Calcium Oxide Content (% by weight) from an assay or published composition so the calculator knows how much CaO is available to weather.
3. Enter Magnesium Oxide Content (% by weight) so the magnesium-bearing portion of the rock is counted in the estimate as well.
4. Run the calculation, then repeat it with a second rock source or a different reaction fraction to see how much the ERW estimate changes before you rely on it.

Formula: estimating CO₂ removal from enhanced rock weathering

The calculation is built around three inputs: applied rock mass, CaO percentage, and MgO percentage, with the reaction fraction acting as the final scale factor. The displayed expression below is the same equation the calculator applies in the browser, written in MathML so the symbols remain readable without changing how the page works. The full formula is shown first, followed by the symbols and conversion factors that appear in the same stoichiometric relationship.

$$C=M\times (\frac{CaO}{100}\times \frac{44}{56}+\frac{MgO}{100}\times \frac{44}{40})\times R$$ In this notation, $M$ is the rock mass, $CaO$ is the calcium oxide assay, $MgO$ is the magnesium oxide assay, and $R$ is the reacted fraction entered on the form. The two stoichiometric conversion factors are grouped together here as $\frac{44}{56},\frac{44}{40}$, which is the same CaO- and MgO-to-CO₂ relationship used by the calculator logic.

The formula is deliberately simple so the calculator stays useful for quick planning. It does not try to model particle size distributions, soil mixing depth, transport emissions, or the way local geology changes dissolution rates from one field to the next. For that reason, the output is best treated as a chemistry-based starting point for ERW project design. If you are comparing two feedstocks, the most important question is usually not whether the formula works, but which input changes the result most and whether the reaction assumption is realistic for the site.

Limitations and assumptions for enhanced rock weathering estimates

This tool is a planning estimate for enhanced rock weathering, not a full field model of every soil type, climate, and handling scenario. The estimate depends on correct rock chemistry, a realistic reaction fraction, and inputs entered in the same units the form expects. It does not replace site-specific monitoring, carbon-accounting review, or local rules that can change as project data and permitting requirements evolve. It also should not be read as a promise that the same amount of CO₂ will be removed everywhere the rock is spread.

For the best use of the calculator, treat the result as one line of evidence among several. Check whether the assay is current, whether the feedstock has already been beneficiated, whether the planned application rate matches the project design, and whether the reaction fraction reflects local moisture and particle size. Those checks do not make the estimate perfect, but they do make it much more informative. In practice, the most honest result is the one that is transparent about what the formula counts and what it leaves out.