Magic Number Calculator
The formula
What the magic number tells you
David Skok's growth-spending test: each S&M dollar last quarter bought how many annualized revenue dollars this quarter? Above 1, the machine compounds.
Calculation: Magic number = (ΔARR × 4) ÷ prior-quarter S&M spend. Enter your inputs and every substitution is shown below the result.
Reading the result
It is a marginal, not average, measure — the decision it informs is the next hiring cohort, not the last one.
Rules of thumb: >1.5 scale aggressively; 0.75–1.5 measured; <0.75 stop and fix
Limitations and best practice
Long sales cycles make it whipsaw; use a two-quarter average when cycles exceed 90 days.
This calculator gives an estimate based on standard formulas; it is not professional financial, medical, legal or engineering advice. Verify anything you plan to act on with a qualified professional and your own records.
Frequently asked questions
Magic number vs CAC payback?
Why 0.75 as the middle line?
Source & methodology
Skok, "The SaaS B2B startup metric: The Magic Number" (2009).
Magic Number: what this tool answers
This page was written for one job: turning your own inputs into a defensible figure, with the working shown. Searchers land on a Magic Number page for three reasons — to plan, to compare options, and to check that somebody else’s arithmetic is honest. This tool serves all three, and everything after the calculator explains the model instead of hiding it.
Like most useful measures, Magic Number is a summary, and summaries hide as much as they reveal. The companion outputs and the input table are here to reopen what the headline compressed.
What makes Magic Number worth calculating is not precision but position: it tells you which side of a threshold you are on, and by how much. A rough figure that settles the direction beats an elaborate one that never commits.
Every figure in the prose is generated by the engine rather than written beside it — the worked example resolves to magic number = 0.77 on the default inputs, and the sensitivity section ranks the inputs that move the result — Current quarter ARR first, at ≈326.8 % of the headline for a ten-percent error either way in that input — which is the thread the rest of this guide pulls on.
Inside the Magic Number calculation: formula and every input
Here is the complete model this tool runs — Magic number = (ΔARR × 4) ÷ prior-quarter S&M spend — with each input explained below. If a competitor’s page gives you a different answer from the same inputs, this is the text that settles which of you is right.
Current quarter ARR ($) (example value 15,525,000) is a plain measured quantity — the unit shown in the label is the unit the formula expects. The example value is there to demonstrate the mechanics, not to describe your situation — overwrite it.
Prior quarter ARR ($) (example value 15,050,000) is a measured number in the unit named by the label; the formula does not convert for you. The example value is there to demonstrate the mechanics, not to describe your situation — overwrite it.
Sales & marketing spend in quarter ($) (example value 2,475,000) is a measured number in the unit named by the label; the formula does not convert for you. If your source quotes a figure before and after some deduction, decide which one the formula wants and be consistent for every field.
Magic Number with real numbers, step by step
Rather than describe the calculation, here it is end to end — every substitution in order, starting from the inputs above. Starting figures: Current quarter ARR ($), Prior quarter ARR ($), Sales & marketing spend in quarter ($). The endpoint of that chain is 0.77 of magic number, sitting in the “Spend more, but measure quarterly” band.
- ΔARR × 4 = $1,900,000
- ÷ $2,475,000 S&M = 0.77
Net new ARR, annualized — $1,900,000. Compare it against the main result of 0.77 before drawing conclusions.
S&M as % of current ARR — 63.8 %. This is the number experienced users quote when the headline alone would mislead.
Next-quarter S&M supported at 1.0× — $1,900,000. If your inputs change, this is usually the first companion figure to move.
Overwrite any input above and every line of this example updates with it — the substitution is live, not a screenshot of somebody else’s numbers.
Scenario table: Magic Number at ±10 % and ±20 % of the key input
Single-point answers invite false confidence, so the same engine was run across a band around the input that matters most here. Each row is a full recalculation of Magic Number, not an interpolation.
| Current quarter ARR setting | Magic number |
|---|---|
| 80 % of the example value | -4.25 |
| 90 % of the example value | -1.74 |
| example inputs | 0.77 |
| 110 % of the example value | 3.28 |
| 120 % of the example value | 5.79 |
Across 5 settings of Current quarter ARR, the result spans -4.25 to 5.79. A decision that only works at one row of this table is a decision waiting for an input to move. Reproduce it in the form above by editing Current quarter ARR alone; the substitution steps will follow your figures exactly.
The levers behind Magic Number, ranked by effect
Nobody should have to guess which input matters. The engine was re-run once per field below — ten percent up, ten percent down, everything else frozen — and the results ranked by how far they moved the magic number. It is the fastest honest answer to “what matters most in Magic Number?” — and it is specific to these defaults, because sensitivity moves with the starting point.
- Current quarter ARR +10 % → Magic Number = 3.28 · −10 % → -1.74 (the magic number shifts ≈326.8 %)
- Nudge Prior quarter ARR by ±10 % and Magic Number lands between -1.66 and 3.20 (sensitivity ≈316.8 %)
- Sales & marketing spend in quarter +10 % → Magic Number = 0.70 · −10 % → 0.85 (the magic number shifts ≈11.1 %)
Current quarter ARR is the dominant lever here: ten percent either way moves the result by about 326.8 %, from -1.74 to 3.28. Measure that input from a document rather than from memory, and Prior quarter ARR is effectively tied with it (≈316.8 % for the same nudge), so verify the pair together. The rest of the inputs earn only a rounding-error of attention, and that asymmetry is the practical payoff of measuring rather than asserting.
Reading the result: bands, zones and thresholds
A number on its own is trivia; a number against a range is intelligence. On the example inputs the band reads “Spend more, but measure quarterly”; your own figures place you on a different rung of the same ladder. Before treating the band as fact, note its width: ten percent either way on Current quarter ARR already spans -1.74 to 3.28. The practical reading rules: compare against your own history before comparing against other people; treat band boundaries as zones, not walls — one point on the wrong side of a threshold rarely changes a real outcome; and when a companion output contradicts the headline, believe the pair.
Where Magic Number sits next to similar measures: this tool answers “how much is the Magic Number”, while the related calculators answer “what do I do about it”. Reading the result alone tends to over-weight a single snapshot; pairing it with the companion tools above turns the number into a decision.
Common mistakes with Magic Number (and the fix)
Long sales cycles make it whipsaw; use a two-quarter average when cycles exceed 90 days. This calculator gives an estimate based on standard formulas; it is not professional financial, medical, legal or engineering advice. Verify anything you plan to act on with a qualified professional and your own records.
Copying a single headline number without checking the companion outputs. The tool reports Net new ARR, annualized, S&M as % of current ARR and Next-quarter S&M supported at 1.0× alongside the main result precisely because Magic Number in isolation misleads — the secondary columns are where outliers and trade-offs show up.
This model has its own particular trap: Current quarter ARR carries ≈326.8 % of the response per ten percent of error, so effort spent elsewhere is largely decorative.
Another is comparing across contexts: a Magic Number computed with one set of conventions should never be lined up against a figure computed with another. Check the inputs behind both before ranking them.
Magic Number verification checklist
- Confirm the example values are plausible for your own situation before replacing them.
- Compare the output against one published benchmark or a colleague’s independent estimate.
- Put Current quarter ARR under a magnifier; the sensitivity table above is the reason why.
- Recompute once with the dominant input deliberately set low, and see whether the decision changes.
- State, in one sentence, which convention each rate, unit or period in the form is using.
- Repeat the ±10 % test on the inputs that matter and see whether the decision — not just the number — still holds.
Step by step: getting a trustworthy Magic Number out of this tool
The tool does the arithmetic; the routine below is what makes the arithmetic trustworthy. Five steps, a minute total — and afterwards you will know not just what the number is, but which assumption produced it.
Step 1 — one document, one pass. Open the statement, label or record that holds current quarter arr ($), prior quarter arr ($), sales & marketing spend in quarter ($) and transcribe straight into the form. Transcription from a single source removes the mismatch errors that appear when figures are assembled from three different places. If a result feels obviously right, that is the moment to check the inputs hardest.
Step 2 — compute the version you would not be embarrassed by. If a stranger audited your inputs, which set would you hand over? Enter that set, read the result, and treat everything else as upside. The test of a good estimate is not that it feels right but that it changes what you do.
Step 3 — isolate your variables. Change one field, observe, revert. The live sensitivity list does the ranking for the default inputs; verify it against your own, because dominance shifts when the starting point shifts. A result worth acting on survives having its two shakiest inputs halved.
Step 4 — run the scenario trio. Expected, pessimistic, optimistic. The gap between pessimistic and expected is the number to plan against; the optimistic result is useful only as a ceiling. The point of transparency is that a reader can disagree with a specific line rather than with you.
Step 5 — record the context, not just the number. Note today’s date and the inputs alongside the 0.77 output. Six months from now the context is what turns this figure into a trend. A figure nobody can reproduce is a figure nobody will act on twice.
Who actually uses Magic Number — and when
Most Startup Metrics disagreements are arithmetic disagreements in disguise. When Magic number of 0.77 sits on the table with its working shown, the conversation stops being about who sounds more confident and starts being about which input is wrong. Numbers are cheap to produce and expensive to retract; the extra minute is the cheapest insurance available.
Used properly this is a five-minute filter that saves weeks. Compute Magic Number, reject what fails, and only then bring in the expensive analysis for the survivors. Repeating a calculation with the same numbers proves only that the arithmetic is reliable — which is worth knowing.
Re-running this page costs nothing and catches the slow drifts that never announce themselves. In Business & Entrepreneurship the expensive mistakes are rarely sudden; they are small changes in Current quarter ARR ($) that nobody recalculated for a year. The input you reach for first is usually the one you measured longest ago.
Quick answers: what searchers ask about Magic Number
What affects Magic Number the most?
The short answer: Current quarter ARR. The longer answer is the sensitivity table on this page, where every numeric input was probed by ten percent and the magic number recalculated — Current quarter ARR moved it furthest, by roughly 326.8 % (-1.74 to 3.28).
Do prior quarter arr and current quarter arr matter equally?
Effectively. Measured on this engine they land at ≈326.8 % and ≈316.8 % for the same ten-percent nudge, so the honest advice is to get both right; chasing precision in only one of them buys almost nothing.
What is a typical Magic Number?
With the example inputs on this page the result is 0.77, labelled “Spend more, but measure quarterly”. “Typical” varies with situation, which is why every result here ships with its verdict band and its sensitivity list: they turn one figure into a range you can locate yourself inside.
How do I calculate Magic Number by hand?
Use the stated model — Magic number = (ΔARR × 4) ÷ prior-quarter S&M spend — and substitute in the order the tool prints (it starts from ΔARR × 4 = $1,900,000). On the example inputs the hand route lands on 0.77, exactly what the calculator shows. A spreadsheet reproduces it in two or three cells; doing it once by hand is what makes the automated answer trustworthy.
What happens to Magic Number if current quarter arr changes by 10 %?
Try it in the form above and the printed substitution will follow you; the pre-computed answer for the defaults is -1.74 to 3.28 across the ten-percent probe, i.e. ≈326.8 % sensitivity on the magic number.
Is the Magic Number result of 0.77 good or bad?
Good and bad are comparisons, so the honest test is three questions: where does your own Magic Number sit against the 0.77 reference above, how far would a plausible input error move it (≈326.8 % per ten percent on Current quarter ARR), and what does the alternative option compute to?
What affects Magic Number the most?
Sensitivity depends on where you start, but the inputs Current quarter ARR ($) and Prior quarter ARR ($) typically move Magic Number further than the rest because they enter the formula multiplicatively. If you can only measure two things well, measure those and hold the others at conservative defaults.
How do I calculate Magic Number manually?
Write down the inputs, then follow the printed steps in order — Magic number = (ΔARR × 4) ÷ prior-quarter S&M spend. A spreadsheet replicates it with two or three cells. The manual route is worth doing once to build intuition; after that the calculator saves time and eliminates transcription slips.
Magic number vs CAC payback?
Payback looks per-customer; the magic number looks at the whole motion including brand and inbound — better for spend decisions.
Why 0.75 as the middle line?
Below 0.75 spending shrinks growth efficiency; 0.75–1.5 is the measured-acceleration zone Skok describes.
Can this tool see the numbers I type?
It cannot. There is no submission step: the calculation happens in your browser and the result disappears when you close the page. Treat the fields above as a scratchpad rather than a form.
Key terms on this page
- Net new ARR, annualized
- $1,900,000 for the Magic Number worked case; it sits next to the main result because it carries context the headline leaves out.
- S&M as % of current ARR
- 63.8 % in the default Magic Number calculation — one of the secondary figures an experienced reader checks before trusting the primary number.
- Next-quarter S&M supported at 1.0×
- $1,900,000 in the default Magic Number calculation — one of the secondary figures an experienced reader checks before trusting the primary number.
- Magic number
- The primary output of this tool: 0.77 for the default inputs, computed exactly from Magic number = (ΔARR × 4) ÷ prior-quarter S&M spend.
- Worked example
- Every substitution in order, from the example inputs to 0.77 — the audit trail for the headline figure.
- Verdict band
- A named zone on the range of plausible results; the example inputs land in “Spend more, but measure quarterly”, and your inputs may not.
- Sensitivity (±10 %)
- How far the Magic Number moves when a single input is nudged ten percent either way; the ranked list on this page is computed live from this engine.
Method, sources and what this tool cannot tell you
The model is published arithmetic implemented exactly as written: Magic number = (ΔARR × 4) ÷ prior-quarter S&M spend. Source of record: Skok, "The SaaS B2B startup metric: The Magic Number" (2009). Any other implementation of this model should return the same figure for the same inputs. If one does not, the difference is in its assumptions — which this page prints and that one probably does not.
Treat the output as a precise answer to the question you typed: exact arithmetic, transparent formula, published source, no hidden rounding. The design constraint was set by usefulness rather than by comfort: an estimate you can interrogate beats an answer you have to trust.
After the number: what to do with it
Re-run the calculation with your own figures, stress the two most sensitive inputs by ±10 %, and only then act on the result.
Whoever you are (people who need one trustworthy number before their next decision), the next step is the same: replace the example inputs with your own, note the date, and re-run whenever a figure moves. In Business & Entrepreneurship, a dated figure is a trend, and an undated one is a rumour.