A calculator is only useful if its answer is right. This page explains how we test every Kesef tool, where the figures inside them come from, and what to do if you think we got something wrong.
How the tools are tested
Each tool has its own proof: a separate script that works out the expected answers with an independent formula, written apart from the tool's own code. The script opens the real tool page in a browser, types in a set of test cases, reads the numbers the page shows, and compares them with the expected answers in a table of expected against actual.
The test cases include the tool's default numbers, realistic family situations, and edge cases such as zero or negative income, very large amounts and unusual settings. A tool passes only when every row matches. We run the proofs again after any change to a tool and before the site is published.
An example
Here is an excerpt from the Maaser calculator's proof, as run on November 8, 2026.
| Test case | Figure | Expected | Tool showed | Match |
|---|---|---|---|---|
| Default numbers | Monthly maaser | $500 | $500 | Yes |
| Default numbers | Total owed for the year | $6,000 | $6,000 | Yes |
| $96,000 a year, $1,200 business costs, 20%, $5,000 extra income, $9,000 given | Monthly maaser | $1,360 | $1,360 | Yes |
| Same case | Still to give | $8,320 | $8,320 | Yes |
| 12.5%, 7 months, $20,000 already given | Ahead by | $13,425 | $13,425 | Yes |
| Income typed as −$5,000 | Monthly maaser (treated as $0) | $0 | $0 | Yes |
Where our figures come from
Tax limits, deposit insurance, contribution caps and similar rules come from primary sources: the agency that sets the rule, not a news story or another calculator. Each figure in a tool's code carries a comment with the date it was checked and the address of the source. Each tool page shows when its figures were last checked, with links to the sources.
The sources we use most:
- IRS: retirement, HSA and 529 limits, standard deductions, tax brackets and charitable deduction rules.
- Social Security Administration: the wage base and cost-of-living adjustments.
- FDIC and NCUA: deposit insurance.
- Consumer Financial Protection Bureau and FTC: credit reports, card and bank account protections.
- State 529 plans and state tax agencies, for state-specific rules.
- Banks' and card issuers' own published terms, where a tool names a product. These change often, so we date them and treat them as examples.
When rules change
Limits are adjusted every year, and laws change in between. When a figure changes, we update the tool, rerun its proof, and put the new check date on the page. The IRS usually announces the next year's retirement limits in the autumn, and we review them when they are published. Changes we think readers should know about go into Numbers that changed, our short monthly email.
Illustrations and assumptions
Some answers depend on things nobody knows in advance, such as investment returns, tuition increases or what a car will be worth in five years. In those cases the tool uses a stated assumption, which you can change, and labels the result as an illustration. Investment returns are never certain and can be negative. Tuition, simcha and Yom Tov defaults are starting points to replace with your own numbers.
How guides are checked
Every figure in a guide links to its source, either in the text or in a sources list at the end, and each guide shows the month it was last updated. Worked examples are our own calculations, and we say so. Where a question is halachic, such as how to calculate maaser, guides describe the common approaches and send you to your rav. We don't rule.
What the tools are not
Kesef is general education. The tools and guides are not financial, tax, legal or halachic advice, and they don't know your full situation. Use them to understand the numbers and prepare good questions. Then ask a qualified professional about decisions that matter, and your rav about halachic questions.
The tools run in your browser. What you type into them is not sent to us. Our privacy policy explains what the site does collect.
Report a mistake
If a number looks wrong, a rule has changed, or a source no longer says what we quote, please tell us through the contact form. Include the page, what you entered and what you expected to see. We check every report against the primary source, fix the tool or guide if we were wrong, rerun the proof and update the date.