About PennyWise Calcs

PennyWise Calcs provides free personal finance calculators with one difference from most of the internet: we show our work. Every calculator page explains the formula it uses, the assumptions behind the defaults, and the honest limitations of the result. The site launched in September 2026 and now covers 55 tools across compound growth, loans, taxes, retirement, insurance, and housing decisions.

Who Runs This Site

My name is Yongwen Wu. I am an independent developer, not a company, and I have worked with data and systems for more than twenty years, first in enterprise systems and later in data engineering, which is a long way of saying I have spent a career making sure numbers arrive from where they came and survive the trip. I am not a licensed financial advisor, and this site never pretends otherwise. What I bring is the other half of the deal: careful arithmetic, named assumptions, and sources you can check yourself.

The site grew out of a habit. When I wanted to answer a money question, refinance math, the break-even age on Social Security, what a Roth conversion actually costs, I would find calculators that gave an answer without showing the math, or articles that showed the math without computing anything. Building the tool and the explanation on the same page fixed that for me, and publishing them seemed like the obvious next step.

How Every Calculator Is Built

Each calculator goes through the same four steps before it goes live.

• Source the parameters. Every fixed input, tax brackets, standard deductions, the Social Security wage base, comes from an official document such as an IRS Revenue Procedure or an SSA fact sheet. Each calculator page ends with a Sources section that links to the exact document behind every key parameter, so a reader can check any figure without leaving the site.
• Build the model twice. The logic is implemented once in the page's JavaScript and once in an independent reference model. If the two ever disagree, the page does not ship.
• Test against worked examples. Every figure that appears in the explanation text is checked against the reference model before it is written down. Defaults are chosen to be realistic rather than flattering, and all of them are overridable.
• Run the full regression. A test harness of more than one hundred automated checks runs against the whole site before each deployment, covering both the math and the rendered pages.

Our Standards

• Transparent math. The formula for every calculation is printed on the page. If we cannot explain it, we do not publish it.
• Conservative defaults. Where a default assumption is required, we choose realistic mid-range values rather than optimistic ones.
• No data collection. All calculations run locally in your browser. Nothing you type is sent anywhere.
• No fake precision. Financial projections over decades are estimates, not promises. We say so on every page.
• Named sources. Government figures cite the issuing agency and document. When we cite a statistic from elsewhere, the organization behind it is named.
• Written editorial rules. What we will and will not publish, how a figure is sourced, and what happens when one turns out to be wrong are set out in the editorial policy.

How the Site Is Funded

PennyWise Calcs is free and carries no paywall. The planned revenue source is ordinary display advertising, which is why you will see ad code referenced in the page source. Calculations themselves never depend on anything external, and no result on this site is influenced by any advertiser, because there are no advertisers, only a general ad network.

What We Are Not

PennyWise Calcs publishes calculators and plain-language summaries of published regulations. Nothing on this site is tax, legal, or investment advice, and no calculator here replaces a licensed professional who knows your full situation. Calculators are educational tools that show the arithmetic consequences of assumptions you choose. For decisions involving large sums, taxes, or legal obligations, talk to someone qualified.

Where Else We Publish

Longer walkthroughs and notes on new calculators go out on Substack and on our Blogger page. Both are written by the same person who builds the calculators, and both link back to the tool being discussed.

Contact and Corrections

Found a bug or a number that looks wrong? We fix errors fast and credit reports. Write to [email protected], or use the details on our contact page. Correction history is part of the methodology page, and the rules for sourcing, review, and correction are in the editorial policy. For the rules of the road, see our terms of use and privacy policy.