MakeItRun blog
Free Calculators Built with Claude (And Live Online in Minutes)
TL;DR — Single-purpose calculators are among the most searched tools on the web. Claude writes them in one prompt. The six live calculators below — coffee ratios, dog age, sleep cycles, pizza dough, cat age, and startup dilution — were each built with a Claude conversation and published to a real URL with MakeItRun in under a minute. No developer required. You can do the same thing today, for free.
Search "coffee ratio calculator" or "dog age in human years" and you get a results page full of tiny, single-purpose tools. No long articles. No onboarding. Just: enter a number, get an answer.
These micro-tools are some of the most consistently Googled things on the internet. People want them quickly, they want them to work on mobile, and they don't care who built them. What matters is that the answer is right.
Claude is exceptionally good at building exactly this kind of tool. Give it a clear prompt and it produces a complete, self-contained HTML file — logic, layout, and styling — in one shot. The only question used to be: now what? How do you get that file onto the internet with a real URL someone can actually visit?
That's what MakeItRun does. Paste the HTML, get a live URL and a QR code. Done.
Here are six calculators we built that way, all running free in your browser right now.
Coffee Ratio Calculator
This one calculates coffee-to-water ratio by brew method. Pick pour-over, French press, AeroPress, or cold brew, enter how much coffee or water you have, and it tells you exactly how much of the other you need — in grams, not tablespoons.
The search terms that bring people here: "coffee to water ratio pour over," "how much coffee for French press," "AeroPress ratio grams."
Coffee nerds use it every morning. Specialty cafes print the QR code and stick it on the counter so customers stop asking the barista.
Claude built the whole thing from a prompt asking for a brew-method selector with gram-based ratio logic. Took one conversation. Published with MakeItRun in seconds.
Dog Age Calculator
The "multiply by 7" rule is wrong. It was always wrong. A 2020 epigenetic study mapped dog aging to human aging by looking at DNA methylation patterns — and the relationship is logarithmic, not linear. A 1-year-old dog is developmentally closer to a 30-year-old human than a 7-year-old one.
This calculator uses that modern formula, with one refinement: size matters. Small dogs age differently than large breeds. You pick the dog's size category and enter the dog's age in years; it returns the human-age equivalent.
Searches that land here: "how old is my dog in human years," "dog age calculator by breed size," "dog years to human years."
The formula itself is public research. Claude implemented it cleanly, added the size selector, and styled a simple card layout. One prompt.
Cat Age Calculator
Same idea, different species. Cats have their own aging curve — rapid in the first two years (a 2-year-old cat is roughly equivalent to a 24-year-old human), then slower and more linear after that.
"How old is my cat in human years" gets searched constantly. Pet owners want to know if their cat is middle-aged or elderly, especially when thinking about health checkups and diet.
This tool gives them a clean answer. No ads, no signup, no scrolling past three paragraphs of background. Just: your cat's age in years → their human-equivalent age.
Sleep Calculator
Sleep cycles run roughly 90 minutes. Waking up mid-cycle — when you're in deep sleep — is why your alarm leaves you foggy even after 7 hours. Waking up at the end of a cycle, even after fewer hours, feels dramatically better.
This calculator works both directions. Enter what time you need to wake up and it tells you the best bedtimes. Or enter when you're going to bed and it shows you the ideal alarm times. It accounts for the average 14 minutes it takes to fall asleep, so the times it suggests are real, not theoretical.
The searches: "what time should I go to sleep," "sleep cycle calculator," "best time to wake up."
There's nothing technically complicated here — it's arithmetic over 90-minute windows. Claude wrote it in one pass. Clean, fast, no cruft.
Pizza Dough Calculator
Baker's percentages are how serious pizza makers think about dough. Everything is expressed relative to the flour weight. Water at 65% hydration means 650g of water per 1000g of flour. Salt runs around 2.8%. Yeast varies with ferment time — same-day proofs need more than a cold overnight.
This calculator takes your flour amount (or your desired number of dough balls) and spits out the exact gram weights for water, salt, and yeast — adjustable for your hydration target and ferment style.
Searches: "pizza dough calculator grams," "baker's percentage calculator," "how much water for pizza dough."
Home pizza obsessives use it every weekend. A few pizzerias have printed the QR code for their staff.
Startup Dilution Calculator
Every time a startup raises money, the founders' ownership percentage shrinks. It's not complicated math, but doing it across multiple rounds — seed, Series A, Series B, option pool shuffles — gets confusing fast.
This calculator models founder equity across funding rounds. Enter your current ownership percentage, the round size, the pre-money valuation, and any option pool expansion; it shows you what your stake looks like after.
Founders use it to sanity-check term sheets. First-time employees use it to understand what their options are actually worth at different exit scenarios.
Searches: "founder equity dilution calculator," "how much equity do I have after funding," "seed round dilution."
One prompt. One HTML file. Live online.
Build Your Own in Three Steps
Every calculator above started with a conversation in Claude. Here's the pattern.
Step 1: Prompt Claude.
Be specific about the inputs, the formula, and the output format. A vague prompt gets a vague result.
Example prompt:
"Build a single-file HTML calculator for body surface area (BSA)
using the Mosteller formula. Inputs: height in cm and weight in kg.
Output: BSA in m² rounded to two decimal places, plus a short note
on what the number means clinically. Mobile-friendly layout. No
external libraries. Clean sans-serif font."
Claude returns a complete HTML file. Copy it.
Step 2: Publish with MakeItRun.
Go to app.makeitrun.ai. Paste the HTML. Click publish. You get a live URL and a QR code — PNG and SVG, ready to drop into any design file or print-on-demand order.
Step 3: Share the link or QR.
Text it, post it, embed it, stick it on a product label. The calculator is live immediately. No server to maintain, no developer to call, no hosting bill to worry about.
If Claude's first output isn't quite right, iterate in the same conversation. "Make the inputs larger on mobile" or "add Celsius as an option" — Claude adjusts and you re-publish. The whole loop takes minutes.
FAQ
Can I build a calculator without coding?
Yes. You describe what the calculator should do in plain English and Claude writes the code. You don't need to read or edit a single line of it. If something looks wrong, describe the problem to Claude in plain English and it fixes it.
How do I put a calculator online for free?
Build the HTML in Claude, then publish it with MakeItRun. The free tier gives you a live URL immediately. No server, no deployment pipeline, no domain required.
Is it really free?
The calculators on this page are free to use with no account required. Publishing your own tools with MakeItRun has a free tier — check makeitrun.ai for current plan details.
Can I make a QR code for my calculator?
Yes, automatically. Every tool published through MakeItRun gets a QR code (PNG and SVG) you can download and use however you want — print menus, product packaging, event signage, wherever.
Can Claude build the calculator for me?
Yes. That's the whole point. Claude writes the HTML, CSS, and JavaScript. You don't need to know how any of it works. Describe the inputs and what you want to calculate, and Claude handles the rest. MakeItRun gets it online.
Put it online with a live URL and a QR code in seconds.
Publish your page →