Skip to content
Free Tools Galaxy
🎲
100% free · no signupRuns in your browser

Probability Calculator

Probability, complement and odds for any event.

Best for: Games and risk, combining the odds of several conditions, and sanity-checking statistical claims.

Complete guide
Why this result happened View assumptions

Input

How it's calculated & sources
The Probability Calculator uses the standard, deterministic formula, so the same inputs always produce the same result. It runs entirely in your browser, so your inputs are never uploaded. Last reviewed June 2026.
Key assumptions
  • Equally likely outcomes

Free & no sign-up · runs entirely in your browser. Results are estimates for general information, not professional advice — verify important decisions with a qualified expert. Last reviewed June 2026.

How it works

P(event) = favorable outcomes / total outcomes. Complement P(not E) = 1 − P(E).

Example

Rolling a 6 on a die: 1/6 ≈ 0.1667 (16.67%)

Frequently asked questions

What is odds?+

Odds = P / (1 − P). A probability of 0.25 is 1:3 odds.

Continue your journey

Where people usually head next.

📏
Significant Figures Calculator
Count significant figures in a number and round to a chosen number of sig figs.
🎯
Percent Error Calculator
Measure how far a measured value is from the true value, as a percentage.
🔬
Scientific Notation Converter
Convert numbers between standard decimal form and scientific (and E) notation.
🧪
Scientific Calculator
Evaluate expressions with sin, cos, log, ln, sqrt, π and e.

Complete guide

Quick answers

Short, sourced answers to the questions people (and AI assistants) ask most.

What is Probability?
A probability calculator works out the chance of an event and combines the probabilities of independent events using AND and OR rules.
Why does Probability matter?
0 means impossible and 1 means certain; the same value can be shown as a fraction, a decimal or a percentage.
How is Probability calculated?
A single probability is favourable outcomes divided by total outcomes, between 0 and 1. For independent events both happening (AND), multiply: P(A) x P(B). For either happening (OR), add and subtract any overlap: P(A) + P(B) - P(A and B); when the events cannot both occur, the overlap is zero.
What are common probability mistakes?
Assuming events are independent when one affects the other.
When should you use the Probability Calculator?
Games and risk, combining the odds of several conditions, and sanity-checking statistical claims.

What is the Probability Calculator?

A probability calculator works out the chance of an event and combines the probabilities of independent events using AND and OR rules.

How the Probability Calculator works

A single probability is favourable outcomes divided by total outcomes, between 0 and 1. For independent events both happening (AND), multiply: P(A) x P(B). For either happening (OR), add and subtract any overlap: P(A) + P(B) - P(A and B); when the events cannot both occur, the overlap is zero.

What impacts results most

  1. 1.
    Favorable outcomes (High impact)The numerator of the probability.
  2. 2.
    Total outcomes (High impact)The denominator of the probability.

Key assumptions

  • Equally likely outcomes

Two-dice sum probabilities (36 equally likely outcomes)

21/362.8%
32/365.6%
43/368.3%
54/3611.1%
65/3613.9%
76/3616.7%
85/3613.9%
94/3611.1%
103/368.3%
112/365.6%
121/362.8%

Key terms

Independent events:
One outcome does not affect the other.
Mutually exclusive:
Events that cannot both happen at once.
Complement:
The chance an event does NOT happen, equal to 1 minus its probability.

Common mistakes to avoid

  • Assuming events are independent when one affects the other.
  • The gambler's fallacy — past independent results do not change the next one.
  • Mixing up AND (multiply) and OR (add), or double-counting the overlap in OR.

Keywords: probability, odds.

Sources: NIST/SEMATECH e-Handbook of Statistical Methods

Reviewed by the Free Tools Galaxy editorial team · Updated June 2026 · Calculated privately in your browser.

Frequently asked questions

Is the Probability Calculator free to use?+

Yes. Every tool on Free Tools Galaxy is 100% free, runs in your browser and requires no signup.

How accurate is the Probability Calculator?+

Probability Calculator uses the standard probability formula in double-precision arithmetic, so the same inputs always produce the same result and you can verify any figure by hand. It is an educational estimate — real-world outcomes depend on your actual rates, rules and assumptions.

Do you store my inputs?+

No. The Probability Calculator runs entirely in your browser. Nothing is uploaded or saved to a server.

Can I use the Probability Calculator on mobile?+

Yes — the interface is fully responsive and works on phones, tablets and desktops.

What are common mistakes to avoid?+

The most frequent mistake is mixing units. Double-check your inputs use a single, consistent unit before clicking Calculate.

Explore more

Popular tools

📝
Word Counter
Count words, characters, sentences and reading time.
🔤
Case Converter
Convert text to UPPER, lower, Title and Sentence case.
🔁
Text Reverser
Reverse any text instantly.
🧹
Remove Duplicate Lines
Strip duplicate lines from any list.

Explore other categories

🧾
SSI Payment Calculator
Estimate your 2026 monthly Supplemental Security Income (SSI) from your income and living situation.
🏦
PPF Calculator
Project your Public Provident Fund maturity with a full year-by-year schedule.
💼
EPF Calculator
Estimate your Employees' Provident Fund corpus at retirement.
🧓
NPS Calculator
Project your National Pension System corpus, lump sum and monthly pension.
Related searches: correlation coefficient calculator, variance calculator, range calculator

Smart Autopilot

AI-style routing · 100% on-device

Based on this tool, your recent activity and how others chain tools together.

Likely next step

📏Significant Figures CalculatorTop match🎯Percent Error CalculatorMeasure how far a measured value is from the true value, as a percentage.🔬Scientific Notation ConverterConvert numbers between standard decimal form and scientific (and E) notation.🧪Scientific CalculatorEvaluate expressions with sin, cos, log, ln, sqrt, π and e.➕Simple CalculatorBasic arithmetic for two numbers: add, subtract, multiply, divide.½Fraction CalculatorAdd, subtract, multiply and divide fractions with auto-simplification.

Smart recommendations

Next best tool

🔗
Correlation Coefficient Calculator
Pearson r with strength interpretation and R².

Also worth a look

📋
SGPA Calculator
Semester Grade Point Average for a single term.
💓
Target Heart Rate Calculator
Karvonen formula target HR zone for cardio.
❤️‍🔥
Heart Rate Zone Calculator
Five training zones from max heart rate.
📝
Word Counter
Count words, characters, sentences and reading time.

Share & publish

Share & challenge

Twitter Facebook LinkedIn WhatsApp
Share-to-grow kit (Reddit, LinkedIn, Medium)
Reddit post
Title: I built/used this free probability calculator — instant, no signup

Body: Probability, complement and odds for any event. Try it: https://freetoolsgalaxy.com/tools/probability-calculator
LinkedIn post
Probability Calculator on Free Tools Galaxy — Probability, complement and odds for any event. https://freetoolsgalaxy.com/tools/probability-calculator
Medium intro
# Probability Calculator — the free tool I keep coming back to

Probability, complement and odds for any event.

Try it: https://freetoolsgalaxy.com/tools/probability-calculator
Quora answer
The fastest way is to use a free in-browser probability calculator: https://freetoolsgalaxy.com/tools/probability-calculator. Probability, complement and odds for any event.

Embed this tool anywhere

Free embed code for blogs, docs and dashboards. Every embed sends a backlink home — helping you and the galaxy grow together.

HTML iframe
Drop into any site — auto-resizes to its content height, theme-aware.
<iframe src="https://freetoolsgalaxy.com/tools/probability-calculator?embed=1" title="Probability Calculator" loading="lazy"
  style="width:100%;height:600px;border:0;border-radius:16px;background:#0b1020"
  allow="clipboard-write"></iframe>
<script>window.addEventListener("message",function(e){if(e.data&&e.data.type==="ftg-embed-height"){document.querySelectorAll("iframe").forEach(function(f){if(f.contentWindow===e.source){f.style.height=e.data.height+"px";}});}});</script>
<p><a href="https://freetoolsgalaxy.com/tools/probability-calculator" target="_blank" rel="noopener">Powered by Probability Calculator on Free Tools Galaxy</a></p>
Responsive wrapper
Aspect-locked container — good for blog post bodies.
<div style="position:relative;padding-bottom:75%;height:0;overflow:hidden;border-radius:16px">
  <iframe src="https://freetoolsgalaxy.com/tools/probability-calculator?embed=1" title="Probability Calculator" loading="lazy"
    style="position:absolute;inset:0;width:100%;height:100%;border:0" allow="clipboard-write"></iframe>
</div>
WordPress / Markdown
Paste in a custom-HTML block or markdown file.
[Probability Calculator — free online tool](https://freetoolsgalaxy.com/tools/probability-calculator)

<iframe src="https://freetoolsgalaxy.com/tools/probability-calculator?embed=1" width="100%" height="600" frameborder="0"></iframe>
Text link-back
Minimal credit link — best for sidebars and footers.
<a href="https://freetoolsgalaxy.com/tools/probability-calculator" target="_blank" rel="noopener">Try the free Probability Calculator</a>

Share-to-Grow Kit

Pre-written, ready-to-paste posts for every major channel. Help others discover this tool — and earn backlinks.

Title
I built a free pregnancy due date calculator that works instantly in the browser — no signup, no ads in your face
Body
Hey r/[subreddit],

I kept needing a quick pregnancy due date calculator and most options online were slow or behind paywalls, so I built one that:

• Runs 100% in your browser (your data never leaves your device)
• No signup, no email wall, no popups
• Loads in under a second on mobile

It covers the standard pregnancy due date use cases (pregnancy, due, date, baby) and links out to the formula + worked examples if you want to learn the math.

Link: https://freetoolsgalaxy.com/tools/pregnancy-due-date-calculator

Happy to take feedback — what's missing?
Post to a relevant subreddit (e.g. r/InternetIsBeautiful, r/webdev, r/productivity). Avoid spammy subs.
Developer & SEO tools