Calculator widgets for your website
Free to put on any site, commercial ones included. Choose the colours and options, copy one snippet, and paste it wherever the calculator should appear. No account, no fee, no tracking, and nothing to maintain afterwards.
Every widget below is live on this page. Try one before you decide.
Loan calculator
Monthly payment, total interest and the share of every payment that is interest, for a car, a personal loan or anything amortizing.
For dealers, credit unions and personal finance writing, where readers arrive ready to borrow.
Get the codePercentage calculator
The four percentage questions people actually ask, each answered in a sentence as well as a number.
Enormous search demand, and it belongs on almost any page.
Get the codeBMI calculator
Body mass index with the WHO category, a marker on the scale, and the healthy weight range for that height.
For gyms, clinics and nutrition sites.
Get the codeCompound interest calculator
What a balance grows to, with monthly deposits and any compounding frequency, split into what you put in and what the interest added.
For investing and personal finance writers, where the point is showing the effect over time.
Get the codeUnit converter
Length, weight, volume, temperature, area and speed, with the defining factors rather than rounded ones.
For recipe sites, trades and any page with an international audience.
Get the codeBasic calculator
The four operators, percent, square root and memory, with real order of operations and keyboard input.
For any page where a reader needs to work out a number without leaving.
Get the codeMortgage calculator
Monthly payment, total interest and a principal against interest split, with optional tax, insurance, PMI and HOA rows.
For property listings, brokers and personal finance writing.
Get the codeScientific calculator
Trigonometry, logarithms, powers, roots, factorial and memory, with real operator precedence and keyboard input.
For education, engineering and science pages where readers need to work something out on the spot.
Get the codeConcrete calculator
Volume and bag counts for slabs, columns and rings, with a waste allowance and a drawing of the shape being measured.
For builders, hardware retailers and home improvement writing.
Get the codeHow these work
Each widget is a single script and a single div. Paste both into your page and the calculator appears where the div sits. Everything runs in your visitor's browser, so nothing is sent to us, nothing is stored, and no cookie banner is needed on account of the widget.
They are built to survive contact with a real website. Each one renders inside a shadow root with its own stylesheet, which means your theme cannot break the calculator and the calculator cannot leak styles into your page. We test that against a deliberately hostile stylesheet, the kind that sets width and font on every input with !important.
You can put more than one on a page, each with its own colours and settings, and they will not interfere with each other. Two versions of the code are offered: a hosted script that updates automatically when we improve something, and a standalone block that contains everything and stays exactly as you copied it.
The one thing we ask is that the small credit link stays. That link is the only reason these are free.
Adding one to your platform
The snippet is the same everywhere. Where it goes is not, so there is a short guide for each.
Common questions
Are the widgets really free?
Yes, on any site including commercial ones. No fee, no account, no usage limit and no trial. The one condition is that the small "powered by Calculator Castle" link stays visible, which is the only thing we get out of it.
Will they work on WordPress, Wix or Squarespace?
Yes, through any block that accepts raw HTML: a Custom HTML block in WordPress, an Embed element in Wix, a Code block in Squarespace. On plans that strip script tags, use the standalone version of the snippet, which carries everything inside itself.
Do they slow the page down?
Each script is around 12 KB, roughly 4.5 KB compressed over the wire, and loads with the async attribute so it never blocks rendering. They pull in no libraries, no fonts and no images.
Do they track visitors or need a cookie banner?
No. Nothing is stored, no cookie is set and nothing is sent back to us. Every calculation runs in the browser of the person using it, so the widget adds no consent obligation of its own.
Will a widget clash with my site styles?
No. Each renders inside a shadow root with its own stylesheet, so your theme cannot reach into the calculator and the calculator cannot leak styles out. They are tested against a deliberately hostile stylesheet, the kind that sets width and font on every input with an important flag.
Can I put several on one page?
Yes. Every element carrying the widget class becomes its own calculator with its own colours and settings, and they do not interfere with each other.
Can I change the colours to match my brand?
Yes. Set data-accent to any hex colour, switch data-theme between light and dark, and change or remove the heading with data-title. The builder on each widget page writes those attributes for you.
What if I want a calculator that has no widget yet?
Ask on the contact page and it goes on the list. Every calculator on the site is a candidate; these nine are simply the ones publishers asked for most.
Want a widget for a calculator that does not have one yet? Tell us which one on the contact page and it will go on the list.