CRISPR gRNA Stoichiometry Planner for Cas & Guide Volumes

You ordered recombinant Cas9 at 10 µM and your synthetic sgRNA arrived at 100 µM. The protocol says “use 100 pmol Cas9 with a 3-fold molar excess of sgRNA per well.” How much of each do you actually pipette? A CRISPR gRNA stoichiometry planner takes your stock concentrations, the desired molar ratio, and the number of reactions, and returns exact volumes for Cas protein, guide RNA, and buffer — including the dead volume overage so you do not run short on the last well.

The most common mistake is mixing Cas and sgRNA by equal mass instead of molar ratio. Cas9 is ~160 kDa; a typical 100-nt sgRNA is ~33 kDa. Equal mass gives roughly a 1:5 Cas9:sgRNA molar ratio — far from the 1:1 or 1:3 that most protocols intend. Editing efficiency drops when either component is limiting, and excess sgRNA without enough Cas protein does nothing useful because unloaded guide RNA does not cut DNA.

A 1:1 Cas9:sgRNA ratio means every Cas9 molecule gets exactly one guide. In practice, not every sgRNA molecule folds correctly or survives handling, so most protocols call for a 1:1.5 to 1:3 molar excess of sgRNA over Cas9. The extra guide RNA ensures that essentially all Cas9 molecules are loaded and active.

Higher ratios (1:5 or 1:10) are sometimes used for difficult-to-edit loci or when sgRNA quality is uncertain (e.g., in vitro transcribed guide with variable folding). But excess free sgRNA in the cell can bind complementary sequences without cutting, potentially interfering with gene expression at off-target sites. For most experiments, 1:3 is the sweet spot.

When using Cas12a (Cpf1) instead of Cas9, the same stoichiometric principles apply but the guide RNA is shorter (~42 nt crRNA vs. ~100 nt sgRNA), so the MW per guide molecule is lower. The calculator accounts for this if you select the correct nuclease — just make sure you are entering the crRNA concentration, not the tracrRNA.

Editing one well is straightforward. Editing a 12-well plate with three biological replicates per guide — four guides total — means twelve RNP reactions plus controls. Building a per-guide master mix saves time and reduces well-to-well variability from pipetting error.

For each guide, calculate the single-reaction volumes of Cas9, sgRNA, and buffer. Multiply by (number of wells + 1 or 2 extra) to account for pipetting dead volume. Pre-complex the Cas9 and sgRNA in the buffer at room temperature for 10–15 minutes, then aliquot the RNP master mix across the wells. Adding cells or electroporation reagent comes after complexing, not before.

Keep separate master mixes for each guide. If you combine Cas9 with guide A and guide B in the same tube, the Cas9 molecules load whichever guide they encounter first, and you lose control over which locus each well targets. Non-targeting control guides get their own master mix too.

Cas9 alone is an inactive hunk of protein. It needs to bind the sgRNA and undergo a conformational change before it can scan DNA for the PAM site. This complexing step happens spontaneously at room temperature and takes about 10 minutes at micromolar concentrations. Skipping the incubation or rushing it means you are delivering a mix of loaded and unloaded Cas9, which lowers editing efficiency.

The standard protocol: mix Cas9 and sgRNA in a low-salt buffer (like Opti-MEM or the nuclease manufacturer’s reaction buffer) and incubate at room temperature (20–25°C) for 10–15 minutes. Do not incubate on ice — RNP formation is temperature-dependent and slows dramatically below 15°C. Do not incubate at 37°C either, as some sgRNAs degrade faster at elevated temperatures.

After complexing, use the RNP within 1–2 hours. Cas9-sgRNA complexes are stable for a few hours at room temperature but lose activity overnight. If you must prepare RNPs in advance, flash-freeze aliquots in liquid nitrogen and store at −80°C. Avoid repeated freeze-thaw cycles — each cycle costs about 10–20% activity.

I get 0% editing even though the guide was validated. What should I check?
First, confirm that the Cas9 and sgRNA were actually complexed before delivery — not just mixed and immediately electroporated. Second, check the Cas9:sgRNA molar ratio. If you mixed by mass and the sgRNA is much lighter per molecule, Cas9 may be in large excess with most molecules unloaded. Third, verify the guide orientation — sgRNA must target the non-template strand relative to the PAM.

Editing efficiency varies wildly between replicates in the same experiment.
This usually means inconsistent RNP delivery. If you are electroporating, check that cell density is uniform across wells (count cells before plating) and that the electroporation pulse parameters are consistent. If you are lipofecting RNPs, make sure the complexing incubation time was the same for all wells and that you mixed the master mix before aliquoting.

My non-targeting control shows 5–10% indels. Is that real?
Probably not. Low-level “editing” in non-targeting controls is usually sequencing noise or PCR artifacts, especially at homopolymer runs. If the indel rate is consistent across negative controls and below 5%, it is background. If it is above 10%, suspect contamination with an active guide in your RNP prep.

Can I use more Cas9 to compensate for a weak guide?
Up to a point. Doubling the RNP amount can improve editing for guides with moderate activity, but the dose-response curve plateaus quickly. Beyond about 200 pmol Cas9 per well in a 12-well format, you get diminishing returns and increasing toxicity from the electroporation/lipofection itself. A better strategy is to redesign the guide.

Four equations get you from stock tubes to loaded pipette tips:

Units note: when stock concentrations are in µM and volumes in µL, the product is pmol directly (no conversion factor needed). If your sgRNA stock is in ng/µL instead of µM, convert first: µM = (ng/µL × 1,000) / (length × 330) for a single-stranded RNA guide.

Scenario: You are editing HEK293T cells with Cas9 RNPs in a 12-well plate. You have two targeting guides (3 wells each) plus a non-targeting control (3 wells) — 9 wells total, but you are making master mixes for 12 wells (9 + overage). Cas9 stock is 10 µM. sgRNA stocks are each 50 µM. Target: 100 pmol Cas9 per well with a 1:3 Cas9:sgRNA molar ratio. Total reaction volume: 10 µL per well.

Step 1 — Per-well Cas9 volume.
V = 100 pmol / 10 µM = 10 µL. That is the entire reaction volume, leaving no room for sgRNA or buffer. Problem. Either use a more concentrated Cas9 stock or reduce the target to 50 pmol.

Step 2 — Adjust to 50 pmol Cas9.
Vₙₐₛ = 50 / 10 = 5 µL per well.
sgRNA needed = 50 × 3 = 150 pmol per well.
Vₛₑₙₐ = 150 / 50 = 3 µL per well.
Buffer = 10 − 5 − 3 = 2 µL per well.

Step 3 — Master mix per guide (4 wells including overage).
Cas9: 5 × 4 = 20 µL. sgRNA: 3 × 4 = 12 µL. Buffer: 2 × 4 = 8 µL.
Total master mix volume: 40 µL per guide. Make three mixes (guide A, guide B, non-targeting control).

Step 4 — Complex and aliquot.
Combine Cas9 + sgRNA + buffer in each tube. Incubate 10–15 minutes at room temperature. Aliquot 10 µL per well. Add cells or electroporation reagent after complexing. You have 10 µL overage per mix in case of dead volume.