Calvin Cycle ( Light–independent Reactions )

Biology • Photosynthesis and Plant Energy

Written by STEM Calculators Team Published January 7, 2026 Updated February 24, 2026

Calvin cycle — carbon & energy accounting (CO₂, ATP, NADPH)

Convert between CO₂ fixed and carbohydrate potential, and compute ATP/NADPH requirements with an optional efficiency factor.

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Mode

Visualizations appear after Calculate. Hover elements to read values; use wheel zoom + drag pan.

Efficiency factor 0–100%

Models losses. For sugar target mode: more inputs are required when efficiency < 100%. For CO2 fixed/per-turn: net product is reduced when efficiency < 100%.

Standard classroom stoichiometry (defaults used internally)
3 CO₂ → 1 net G3P requires 9 ATP and 6 NADPH. Glucose = 2 G3P; sucrose = 4 G3P.

Product selection

Carbohydrate basis

Used for sugar-target and for reporting sugar potential in CO2-fixed mode.

Input basis

If grams is selected, the calculator converts using molar masses.

Amount

Interpreted in the unit below.

Amount unit

Used for “amount in moles”.

Molar masses used (editable)

These are standard approximate values. You can adjust if your course/lab uses different conventions.

Glucose (g/mol)

Sucrose (g/mol)

G3P equivalent (g/mol)

Glyceraldehyde-3-phosphate (approx.).

CO2 (g/mol)

Sugar target inputs

Enter your target carbohydrate amount above, choose efficiency, then Calculate to get required CO2, ATP, NADPH.

Batch / CSV scenarios (paste or upload)

Paste CSV (comma or tab). Supported columns: mode (sugar/co2/turn), product (g3p/glucose/sucrose), basis (mol/g), amount (for sugar: product amount; for co2: CO2 amount; for turn: turns), unit (umol/mmol/mol; ignored if basis=g), eff (0–100).

CSV input

You can copy/paste directly from a spreadsheet.

Upload CSV file

Batch results appear in the Results table. You can copy/download CSV afterwards.

Results

Run Calculate to see outputs, graphs, and steps.

Calvin cycle schematic (fixation → reduction → regeneration)

Hover each phase to see ATP/NADPH usage. Wheel zooms; drag to pan.

Sankey-style carbon flow (CO2 → G3P → product)

Hover flows to see amounts. Wheel zooms; drag to pan.

Requirement bars (ATP vs NADPH; CO2 reference)

Hover bars for values. Wheel zooms; drag to pan.

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Calvin cycle: what this calculator is modeling

The Calvin cycle (light-independent reactions) is the pathway where plants use ATP and NADPH (made by the light reactions) to convert CO₂ into carbohydrate. For planning and teaching, the most useful part is the carbon and energy accounting: how many moles of CO₂, ATP, and NADPH are associated with a given carbohydrate target.

Key simplification used

Standard textbook bookkeeping (net synthesis): per CO₂ fixed the Calvin cycle consumes approximately 3 ATP and 2 NADPH. This already includes the costs of the regeneration steps.

Three phases in one sentence each

Fixation: CO₂ is attached to RuBP by Rubisco → forms 3-PGA.
Reduction: 3-PGA is reduced using ATP and NADPH → forms triose phosphate (G3P).
Regeneration: most triose phosphate is used (ATP cost) to regenerate RuBP so the cycle can continue.

Stoichiometry summary used by the calculator

Target product	Carbons in product	CO₂ required (mol per mol product)	ATP required (mol per mol product)	NADPH required (mol per mol product)
Triose phosphate (G3P equivalent)	3	3	9	6
Glucose equivalent	6	6	18	12
Sucrose equivalent	12	12	36	24

Where these numbers come from (general formulas)

Let the product contain C carbon atoms per molecule (for example: G3P has C = 3, glucose has C = 6). Each CO₂ contributes one carbon, so:

\[ n_{\mathrm{CO_2}} = C \cdot n_{\mathrm{product}} \]

Using the standard per-CO₂ energy costs:

\[ n_{\mathrm{ATP}} = 3 \cdot n_{\mathrm{CO_2}}, \qquad n_{\mathrm{NADPH}} = 2 \cdot n_{\mathrm{CO_2}} \]

For example, if the target is 1 mol glucose (C = 6), then \(n_{\mathrm{CO_2}} = 6\) mol, \(n_{\mathrm{ATP}} = 18\) mol, and \(n_{\mathrm{NADPH}} = 12\) mol.

How the calculator handles the three modes

Mode 1 Sugar target → requirements

You enter a carbohydrate type and an amount (mol or grams). The calculator converts grams to moles (if needed), then computes CO₂, ATP, and NADPH requirements using the formulas above.

Mode 2 CO₂ fixed → sugar potential

You enter CO₂ fixed and choose which product to express the output as. The calculator converts CO₂ to “product-equivalent moles” by dividing by the carbon count:

\[ n_{\mathrm{product,theoretical}} = \frac{n_{\mathrm{CO_2}}}{C} \]

Mode 3 Per turn (per CO₂)

A “turn” here means fixing 1 CO₂. The calculator reports: \(3\) ATP and \(2\) NADPH per turn, plus optional scaling to any number of turns you enter.

Efficiency factor: modeling losses (0–100%)

Real photosynthesis is not 100% efficient (photorespiration, respiration, limitations in light, CO₂, and temperature). The calculator uses an efficiency factor \( \eta \) between 0 and 1 to scale the theoretical result.

How efficiency is applied in each direction

From sugar target → requirements

If only a fraction \( \eta \) of the theoretical output is realized, you need to “aim higher” in inputs:

\[ n_{\mathrm{CO_2,required}} = \frac{n_{\mathrm{CO_2,theoretical}}}{\eta}, \quad n_{\mathrm{ATP,required}} = \frac{n_{\mathrm{ATP,theoretical}}}{\eta}, \quad n_{\mathrm{NADPH,required}} = \frac{n_{\mathrm{NADPH,theoretical}}}{\eta} \]

From CO₂ fixed → sugar produced

The net carbohydrate potential is reduced:

\[ n_{\mathrm{product,net}} = \eta \cdot n_{\mathrm{product,theoretical}} \]

If \( \eta = 1 \), you recover the textbook stoichiometry. If \( \eta = 0.80 \), you model a 20% loss.

Worked examples

Example A: glucose target

Target: 2 mmol glucose, assume \( \eta = 1 \).

\[ C = 6,\quad n_{\mathrm{product}} = 2\ \mathrm{mmol} \] \[ n_{\mathrm{CO_2}} = C\cdot n_{\mathrm{product}} = 6 \cdot 2 = 12\ \mathrm{mmol} \] \[ n_{\mathrm{ATP}} = 3\cdot n_{\mathrm{CO_2}} = 3 \cdot 12 = 36\ \mathrm{mmol} \qquad n_{\mathrm{NADPH}} = 2\cdot n_{\mathrm{CO_2}} = 2 \cdot 12 = 24\ \mathrm{mmol} \]

Example B: CO₂ fixed → G3P equivalent

CO₂ fixed: 30 mmol. Express as G3P (C = 3). Assume \( \eta = 0.90 \).

\[ n_{\mathrm{product,theoretical}} = \frac{n_{\mathrm{CO_2}}}{C} = \frac{30}{3} = 10\ \mathrm{mmol} \] \[ n_{\mathrm{product,net}} = \eta\cdot n_{\mathrm{product,theoretical}} = 0.90 \cdot 10 = 9\ \mathrm{mmol} \]

Example C: per turn (per CO₂)

For 1 CO₂ fixed (one “turn”):

\[ n_{\mathrm{ATP}} = 3,\qquad n_{\mathrm{NADPH}} = 2 \]

For 12 turns, multiply both by 12: \(36\) ATP and \(24\) NADPH.

How to interpret the visualizations in the calculator

Cycle schematic: badges summarize where ATP and NADPH are used (reduction + regeneration).
Sankey-style flow: CO₂ enters, product equivalents leave; “energy input” arrows summarize ATP/NADPH demand.
Requirement bar chart: quick comparison of ATP vs NADPH demand for the selected product and amount.

Important teaching note

This calculator is for net accounting. It does not simulate metabolite pools, enzyme kinetics, or light-reaction limitations. It is intended for planning, estimation, and classroom stoichiometry.

Common pitfalls

Mixing units: keep track of whether you are entering mmol vs mol vs grams (the calculator converts, but your lab notes should be consistent).
Efficiency: if \( \eta \) is small, required ATP/NADPH can increase quickly because the calculator divides by \( \eta \).
“Glucose vs sucrose equivalent”: sucrose is a 12-carbon sugar (roughly two hexose units), so it doubles carbon and energy requirements relative to glucose.

Frequently Asked Questions

How does the Calvin cycle calculator compute ATP and NADPH requirements from CO2?

It uses standard textbook bookkeeping: per CO2 fixed, the model consumes about 3 ATP and 2 NADPH. For a chosen product, CO2 required is based on carbon count, then ATP = 3 x CO2 and NADPH = 2 x CO2.

What does the efficiency factor change in the results?

Efficiency models losses between 0% and 100%. For sugar target mode, required CO2, ATP, and NADPH increase when efficiency is below 100%; for CO2-fixed and per-turn modes, the net sugar potential is reduced by the efficiency factor.

What does a turn or batch mean in the per-turn mode?

In this tool, a batch represents 3 CO2 fixed producing 1 net G3P equivalent under the simplified stoichiometry. The calculator scales CO2, ATP, NADPH, and product equivalents by the number of batches entered.

Can I enter grams instead of moles for sugar or CO2?

Yes. When grams are selected, the calculator converts grams to moles using editable molar masses for CO2 and the selected carbohydrate basis, then performs the stoichiometric accounting.

How do I run multiple Calvin cycle scenarios at once?

Use the Batch/CSV section to paste a table or upload a CSV with columns like mode, product, basis, amount, unit, and efficiency. After running the batch, the calculator shows a results table and supports copying or downloading the output as CSV.

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Calvin cycle — carbon & energy accounting (CO2, ATP, NADPH)

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Calvin cycle — carbon & energy accounting (CO₂, ATP, NADPH)