Why This Page Exists
A financial calculator is only useful if you can trust its numbers. This page explains how we check ours: how the math is tested, where the figures come from, and how we approach accessibility. Where something hasn't been checked yet, we say so.
How the Math Is Tested
A formula that's right today can drift later, especially when the same math is copied onto several pages. These are the checks that keep it consistent.
One shared set of formulas
Many of our calculators, including the loan, mortgage, savings, emergency fund, paycheck, state income tax and 401(k) calculators, use one shared library of formulas instead of separate copies on each page. Our calculator API uses the same library, and automated tests confirm the two stay identical.
New checks are tested too
A test that has never failed hasn't proven anything. When we add a new accuracy check, we first break the code on purpose to make sure the check catches it.
Old answers checked before new code goes live
When a calculator moves onto the shared formulas, we first record its answers across a wide range of inputs, including edge cases. The new version has to reproduce every one of those answers before it goes live. Three of our embeddable widgets (loan, mortgage payment and savings) have moved over so far.
Measuring the effect of a fix
When we found that the savings widget was compounding interest the wrong way, we measured how much the fix changed results across 150 realistic scenarios and recorded it before the correction went live.
Site-wide browser checks
An automated browser check can run across every page on the site, looking for missing or broken values, script errors, layout problems and missing styles. On each calculator it also runs a calculation to confirm the result updates.
Automated tests, typical inputs to edge cases
Automated tests check our calculations, from typical inputs to edge cases and boundary values. Dedicated checks also confirm that our calculator API and this site produce the same state-tax results. The tests run automatically whenever the code changes.
Where Every Number Comes From
A result is only as good as the data behind it. Here's how we keep track of where each official figure comes from, such as tax brackets, contribution limits and Social Security bend points.
A source record for every official figure
Each tax bracket, contribution limit and wage base our calculators use is stored with its source, a link to that source, and the date it was last checked. Our site build reads and checks these records; they aren't just notes for developers.
Dates that have to agree
We also track when each figure was last verified in our calculator API. If the site and the API disagree about those dates, or a date is in the future, the site isn't updated until the mismatch is fixed.
The "Data sources and verification" panel
On the 401(k), IRA, RMD and Social Security calculators, each result has a "Data sources and verification" panel underneath it. Open it to see the source of each figure used, when it was last checked, and which version of our formulas produced the result.
One calculator without the panel yet
The Roth conversion calculator doesn't show this panel yet. It relies on Medicare IRMAA income brackets that we're re-checking, and we'd rather show no panel than one covering figures that may be out of date.
Check a figure yourself
Six of our methodology pages include a "Check it yourself" section that takes one result back to its primary source, step by step: a mortgage payment, the 2026 401(k) limit, a required minimum distribution, Social Security bend points, savings growth at a stated APY and paycheck withholding.
Sources included for AI assistants
When an AI assistant uses our calculator API, every result comes back with the data sources and rule year behind it, along with the version of the formulas that calculated it. See our AI crawler policy for which AI assistants and crawlers can read this site directly.
Our Accessibility Target
We build to the WCAG 2.1 Level AA standard across the site. That's a target, not a certification, so here's what we do today and what we haven't done yet.
Automated accessibility scanning
We use axe-core, an automated accessibility checker, to test for problems like low color contrast, incorrect ARIA usage, missing form labels, out-of-order headings, and missing image alt text. The scans cover our key pages and most of our calculators.
Tap targets big enough to use
Buttons and links are checked against the 24-pixel minimum touch-target size in WCAG 2.5.8. This year we found and fixed 22 tap targets that were too small in the shared navigation, footer and form controls, so the fix reached every page that uses them.
Works with a keyboard
Calculators and embeddable widgets use standard HTML form controls, which work with a keyboard by default. Every page also has a skip-to-content link so keyboard users can jump past the navigation.
Not done yet: a manual screen reader review
We haven't yet had someone who relies on a screen reader, such as NVDA, JAWS or VoiceOver, go through the site. Automated scans catch many accessibility problems, but not everything a screen reader user would run into.
What's Free Today
This list comes from the same data that builds the site, so it stays current.
40 calculators, no signup
Every calculator on this site is free to use, and none of them asks for an account or an email address.
10 embeddable widgets
10 of our calculators are also available as widgets you can add to your own site with a short snippet of code. There's nothing to sign up for and no card needed; a key is created when you copy the embed code. Three of the widgets (loan, mortgage payment and savings) already use the shared formulas described above, and the others are moving over one at a time.
Attribution cannot currently be removed
Every embed includes a small "Powered by Digital Calculator" link. There is currently no way to remove it at any price — the list of keys allowed to suppress it is empty, and an automated test pins that list at empty so any future change to it would be a deliberate, visible one, not a quiet flip. This is separate from white-label branding on API PDF reports, which is set by hand on an API key and changes the report, not the embed. The embed link can only be removed through a completed White-Label purchase, never with a Pro or Free key, and this section will stay as written until the first such purchase.
No usage cap on embeds
We don't currently limit how often an embedded widget can be used. It works on the honor system, like most embeds that run in the visitor's browser, because enforcing a cap would mean contacting our server on every page load, and our widgets deliberately avoid that.
A free calculator API
Our calculator API is also free to connect to and use today, with 10 tools available and no API key required.
Saved scenarios are free and stay on this device
On our comparison calculators you can save a named set of inputs, load it next to your current one to see what's different, and export the comparison as a file. All of this happens in your browser. Saved scenarios are stored only on this device; they're never uploaded or sent to us, and there's no account to attach them to. You can see and clear them on the Saved on this device page, or with your browser's "clear site data" option. Saving scenarios is free and has no limits.
What We Don't Claim
- This isn't a full financial-planning service. The calculators answer specific questions and don't replace a complete financial plan.
- We haven't had a manual accessibility review with a screen reader yet, only automated scans (see above).
- Not every embeddable widget uses the shared formulas yet. Three do, and the rest are moving over one at a time.
- Not every calculator is available as a widget. A few, including RMD and Social Security, are only available on this site.
Found Something That Looks Wrong?
Email admin@markcolabs.com and we'll check it against the source. If we find an error, we fix it and update the page's date. The About page has more on how we handle corrections.
Important Disclaimer
This page is for general information and education. It isn't financial, tax or legal advice, and your situation may differ, so talk to a qualified professional before making a financial decision. Calculator results are estimates based on the numbers you enter and each calculator's assumptions, and actual outcomes can vary. Tax and benefit rules change often; the figures described here are current as of September 2026.