crux calculator

What is a crux?

A crux is the pivotal assumption behind a decision. If that assumption flips, your decision likely flips too. People often waste time debating surface-level details when the real leverage is hidden in one unresolved question. The goal of this calculator is simple: help you identify whether a specific assumption deserves immediate testing.

Why this matters

In personal finance, career planning, product strategy, and even relationships, we make bets under uncertainty. Most bad outcomes are not caused by laziness; they come from being confidently wrong about one key fact. A crux-first workflow helps you reduce that risk before you commit major time, money, or reputation.

Common examples of crux assumptions

• "If I charge more, customers will leave."
• "This side project can reach profitability in six months."
• "I can sustain this routine long enough to get results."
• "This investment thesis depends on rates falling next year."

How the Crux Priority Score works

The calculator combines upside and friction:

• Upside inputs: impact, decision importance, and chance you may be wrong.
• Friction inputs: testing cost and testing time.

Formula used:
raw = (impact × importance × probabilityWrong) / 10
friction = 1 + (cost / 500) + (time / 20)
cruxScore = min(100, (raw / friction) / 10)

Higher score = stronger reason to test now. Lower score = either low upside or too expensive to test immediately.

How to use it in practice

1) Name one assumption

Keep it singular and testable. Avoid broad statements like "my business model is good." Better: "At least 5% of visitors will book a paid call."

2) Estimate impact and importance honestly

Overrating every assumption makes the model useless. Use 9s and 10s rarely. Most assumptions should land between 4 and 8.

3) Enter your probability of being wrong

This is where humility creates edge. If you think there is a 40%-60% chance your assumption is wrong, it is often worth testing before scaling.

4) Reduce test friction

If the score is low because cost/time are high, redesign the experiment:

• Run a smaller pilot.
• Use a fake-door test or pre-sale page.
• Time-box the experiment to one week.

Score interpretation guide

• 75-100: High-priority crux. Test immediately before making a major commitment.
• 50-74: Important. Schedule a focused experiment soon.
• 25-49: Medium. Track it, but test after higher-leverage unknowns.
• 0-24: Low-priority. Not your current bottleneck.

Worked example

Suppose you're considering a paid newsletter. You estimate:

• Impact if true: 8
• Decision importance: 9
• Probability you're wrong: 50%
• Test cost: $100
• Time to test: 5 hours

The score lands in the "test soon" band. So instead of building a full content system, run a one-week pre-order experiment. That gives you data fast and avoids expensive guessing.

Final thought

Clear thinking is not about having perfect predictions. It is about rapidly finding the assumption that matters most, then testing it at the lowest possible cost. Use this calculator as a weekly planning tool, and you'll make fewer emotional decisions and more evidence-driven ones.