Why This Page Exists
Financial calculators are only useful if you can trust the numbers they give you. Most calculator sites ask you to take that on faith. This page is the opposite of that: it names the actual files, tests, and build gates behind our accuracy and accessibility work, so you can check the claim instead of trusting it. Where we have not yet done the work to back up a claim, we say so directly rather than implying otherwise.
The Audited Core
Calculator math is easy to get right once and easy to let drift afterward — a formula copied onto five pages can quietly diverge from itself over time. Here is what actually prevents that on this site.
One shared engine, not per-page copies
A growing set of calculators — including loan, mortgage, savings, emergency fund, state income tax, paycheck withholding, and 401(k) projections — run on a single shared engine library rather than separate copies on each page. A dedicated test compares the copy our calculator API server uses against the source library byte-for-byte on every change, so the two cannot silently disagree.
Proven to actually fail before we trust it
A test that has never failed hasn't proven anything. When that byte-for-byte comparison shipped, we deliberately introduced a one-line change into a generated copy to confirm the test would catch it — it flagged exactly the one file affected and nothing else. We hold new accuracy guards to that same standard: proven failing first, not just added and assumed to work.
A grid of old answers before a rebase ships
When we move a calculator's math onto the shared engine, we don't swap the code and hope. We first run the old code across a wide grid of inputs, including edge cases, and record every output. The new code must reproduce every one of those values before the change can ship. Three of our embeddable widgets — loan, mortgage payment, and savings — have made this move so far.
Disclosed, not silent, when a number moves
When we found the savings widget was compounding interest using the wrong method, the fix shipped with a dated internal record quantifying exactly how much results moved, across a 150-point grid of realistic inputs, before it went live. A correction with a paper trail is different from a silent diff.
A full-site sweep on demand
We can run an automated browser check across every page this site builds — currently 237 pages, covering all 39 calculators — looking for broken or missing displayed values, uncaught JavaScript errors, layout overflow, and missing stylesheets. On every calculator, it also triggers one live recalculation to confirm the displayed result actually updates.
A test suite that gates every change
As of September 2026, our automated test suite runs 8,600+ individual test cases across nearly 400 test files, plus dedicated guards that compare our calculator API's state-tax math against the site's and fail if they disagree. Every pull request runs the full build and the full test suite before it can be merged — a failing suite blocks the merge, it does not just warn about it.
Where Every Number Comes From
A calculator result is only as good as the data behind it. Here is how we track — and let you check — the source of every statutory figure we use, like tax brackets, contribution limits, and Social Security bend points.
A provenance record on every statutory figure
The statutory constants our calculators use — tax brackets, contribution limits, wage bases — carry a structured record naming the source, a link to it, the date it was last verified, and who verified it. This isn't a comment for developers to read; it's data our build actually checks.
A build that refuses to ship disagreeing dates
A dedicated build step compares that provenance record against a separate freshness-tracking file and against the dates our calculator API publishes for the same figures. If any of the three disagree about which is more recently verified, or if any date is somehow in the future, the build fails outright rather than shipping the contradiction.
The "Data sources and verification" panel
On calculators that compute from statutory rules — 401(k), IRA, RMD, and Social Security — every result includes a collapsed panel below it naming the source for each figure used, the date it was last checked against that source, and the exact version of the engine that produced the result. Open any of those four calculators and look for "Data sources and verification" beneath your result.
Honest limit: one page deliberately excluded
The Roth conversion calculator does not show this panel today. It depends on Medicare IRMAA income brackets that we have flagged internally as needing re-verification, and we would rather show nothing than show a verification panel over data we already know may be stale.
Machine-readable sources for AI assistants
Our calculator API (a Model Context Protocol server that AI assistants can call directly) returns a sources field with every result — a set of reference URIs such as dc://irs/limits/2026 and dc://ssa/bend-points/2026 naming exactly which statutory dataset and rule year produced the answer, plus the engine version that computed it.
Our Accessibility Target
We build to the WCAG 2.1 Level AA standard as our target across every calculator and page. A target is not a certification, so here is exactly what we do today and where we have not yet gathered evidence.
Automated scanning on every calculator
Our automated test suite runs axe-core, an accessibility rules engine, against every calculator page — checking color contrast, ARIA usage, form label associations, heading order, and image alt text. This runs as part of our standard test pipeline, not as a one-time audit.
Touch targets sized against a real standard
We measure and fix touch-target sizing against WCAG 2.5.8's 24px minimum. This year we identified and closed 22 undersized tap targets across shared navigation, footer, and form-control components — fixing the shared component once so the correction reaches every page that uses it, instead of patching pages one at a time.
Keyboard-operable by construction
Every calculator and embeddable widget is built from native HTML form controls rather than custom widgets that can trap keyboard focus, and every page carries a skip-to-content link so keyboard users can bypass repeated navigation.
Honest limit: no manual assistive-technology audit yet
What we have not done: a manual walkthrough by someone using a screen reader (such as NVDA, JAWS, or VoiceOver) as their primary way of accessing the page. Automated scanning catches a meaningful slice of accessibility problems, but it does not catch everything a person actually relying on assistive technology would notice. We are stating that plainly rather than implying a manual audit has happened.
What's Free Today
An inventory re-checked against the same registries that drive this site's build, not copied from a marketing sheet.
39 calculators, no signup
Every calculator on this site is free to use, with no account, no email address, and no paywall on any of them.
10 embeddable widgets
10 of our calculators are also available as widgets you can embed on your own site with one script tag and one div — no signup and no card required. A key is generated automatically the moment you copy the embed code. Three of those widgets (loan, mortgage payment, and savings) run on the audited shared engine described above today; the rest are being migrated onto it one at a time, each behind its own accuracy gate.
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.
No enforced usage cap on embeds
We do not currently meter or cap embed usage. Enforcement today is honor-system, the same posture other client-side embed providers take, because making it hard-enforceable would require a server round trip on every page load, which our widgets deliberately don't make.
A free calculator API
Our calculator API is likewise free to connect to and query today, with 9 tools available and no API key required.
What We Don't Claim
- We are not a comprehensive financial-planning platform. These are calculators for specific questions, not a substitute for a full financial plan.
- We have not completed a manual assistive-technology audit (see above) — only automated accessibility scanning.
- Not every embeddable widget runs on the audited shared engine yet — three do today, and the rest are being migrated one at a time.
- Not every calculator exists as an embeddable widget. A few — including RMD and Social Security — are calculator pages only today.
Found Something That Looks Wrong?
Tell us at admin@markcolabs.com and we will check it against the source. If we find an error, we fix it and note it against the affected page — see our About page for how we handle corrections generally.
Important Disclaimer
This content is for educational and informational purposes only and does not constitute financial, tax, or legal advice. Individual circumstances vary, and you should consult with a qualified professional before making financial decisions. Calculator results are estimates based on the information you provide and the assumptions built into each tool; actual outcomes may vary. While we strive for accuracy and describe our verification process above, laws and regulations change frequently. Data current as of September 2026.