Money decisions, made clear
Plain-English answers and free calculators for the money decisions that actually cost you. Sourced, current, no signup.
Cobalt Prosper is a free reference for everyday money decisions. It holds 26 calculators that show their arithmetic and 86 guides written in plain sentences, plus the property tax and sales tax figures for every US county. Every number is traced back to a primary source, and nothing here is behind an account or an email wall.
Start with a topic
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Taxes
Brackets, deductions, and what you actually owe, with calculators that show the arithmetic.
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Retirement
How much you need, which accounts to use, and how long the money lasts.
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Student Loans
Repayment plans, forgiveness rules, and the real lifetime cost of each option.
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Insurance
How much coverage you need, what it should cost, and what you can safely skip.
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Credit
How scores are built, what moves the number, and how to fix a low one.
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Homeownership
What you can afford, what the payment really is, and whether buying beats renting.
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Auto
Loan, lease, or keep the old one, and what a car actually costs once it is yours.
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Start a Business
The steps, the forms, and the real costs of getting a business off the ground.
Look up your county
Property tax and sales tax are set locally, so a national average tells you very little. Both are here for every county in the country, with the figure, the date it was read, and which level of government sets it.
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Property tax by county
Effective rate and median bill for all 3,144 US counties, from Census data, ranked within each state.
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Sales tax by county
Combined state and local rate for all 3,133 counties, including where the rate changes inside a county.
Calculators
All 26 of them are free, run entirely in your browser, and show the formula they used. Nothing you type is sent anywhere.
Taxes
- 1099-R tax and early-withdrawal penalty calculator
- Pay raise take-home calculator
- Sales tax calculator
- Sales tax economic nexus checker
- Self-employment tax and quarterly estimated tax calculator
- Take-home pay calculator by state
Retirement
- 401(k) contribution calculator for 2026
- Compound interest calculator
- FIRE calculator
- Solo 401(k) contribution calculator
- Solo 401(k) eligibility checker
Student loans
Insurance
- ACA marketplace subsidy calculator
- HDHP vs PPO break-even calculator
- HSA vs FSA savings calculator
- Medicaid eligibility calculator
Credit
Start a business
Homeownership
- How much house can I afford?
- Mortgage calculator
- Mortgage refinance break-even calculator
- Rent vs. buy calculator
Auto
Recently updated
- Sales tax by county Taxes, updated September 20, 2026
- The bona fide residence test Taxes, updated September 15, 2026
- The foreign earned income exclusion Taxes, updated September 15, 2026
- Form 8840 and the closer connection exception Taxes, updated September 15, 2026
- The physical presence test Taxes, updated September 15, 2026
- Resident alien or nonresident alien? Taxes, updated September 15, 2026
- The substantial presence test Taxes, updated September 15, 2026
- What is a tax home? Taxes, updated September 15, 2026
How this site works
Every figure on this site comes from a dataset that is checked before the site builds, and each dataset cites the agency page it was read from. Where a number could not be verified on a primary source, the page prints nothing rather than a guess. Projections are labeled as projections. The full editorial method is on the about page, including who writes this and how corrections are handled.
Free to use. No accounts, no email walls. Every number is traced to a primary source, and projections are labeled as projections.