Guides
Each guide explains the math a calculator uses, the assumptions built into it, and where real-world figures commonly differ. New guides are published only when they add something the calculator itself does not already say.
How Mortgage Payments Are Calculated
The amortization formula behind a fixed-rate mortgage payment, a worked example, what PITI includes, and why your lender quote differs from a calculator.
Open the Mortgage CalculatorHow Much House Can You Afford?
How lenders calculate affordability using debt-to-income ratios, a worked example, and why the maximum you qualify for and the amount you should spend are different numbers.
Open the Home Affordability CalculatorHow Mortgage Refinancing Works
How a refinance replaces your loan, the break-even math on closing costs, cash-out refinancing, and when refinancing usually does not pay off.
Open the Refinance CalculatorHow Much Gravel Do I Need?
Convert length, width and depth into cubic yards and tons of gravel, choose a realistic depth, and understand why tonnage varies by product.
Open the Gravel CalculatorHow Much Concrete Do I Need?
Calculate concrete for slabs, footings and post holes in cubic yards, with two worked examples, mix strength guidance and ready-mix ordering tips.
Open the Concrete CalculatorHow Much Flooring Do I Need?
Measure rooms correctly, choose a waste percentage that matches your layout, and convert square footage into boxes with two worked examples.
Open the Flooring CalculatorHow Much Paint Do I Need?
Calculate paintable wall area, subtract openings correctly, and convert to gallons using real coverage rates, with two worked examples.
Open the Paint CalculatorWhat Are Closing Costs? A Buyer's Guide to the Fees
Closing costs explained for US buyers: every fee category, two worked cash-to-close examples, which charges are negotiable, and how to read your Closing Disclosure.
Open the Closing Cost CalculatorHow Much Should You Put Down on a House?
What different down payment sizes change: PMI, monthly payment, loan-to-value and cash reserves, with two worked comparisons.
Open the Down Payment CalculatorRent vs Buy: What Costs Should You Compare?
A fair rent-versus-buy comparison includes maintenance, closing costs, selling costs, equity and opportunity cost, with two worked examples across different time horizons.
Open the Rent vs Buy CalculatorHow Property Taxes Are Calculated
Assessed value, assessment ratios, millage rates, exemptions and two worked examples of why property tax varies so much between counties.
Open the Property Tax CalculatorHow to Build a Home Renovation Estimate
How to build a home renovation estimate from measured quantities, local quotes, labor share and contingency instead of national averages, with two worked examples.
Open the Home Renovation Cost CalculatorHow to Estimate Mini-Split Cooling Capacity
A step-by-step room cooling estimate using the ENERGY STAR sizing chart and its published adjustments, with a worked example and when to get a Manual J.
Open the Mini-Split BTU EstimatorWhy Square Footage Alone Can Mis-size an Air Conditioner
Two rooms with the same floor area can have very different cooling loads. See which factors matter, why oversizing hurts comfort, and what a Manual J adds.
Open the Mini-Split BTU EstimatorCFM vs ACH: Which Airflow Calculation Do You Need?
Air changes per hour and supply CFM answer different questions. Learn both formulas, convert between them correctly, and avoid using ventilation airflow to size cooling.
Open the Room-by-Room CFM PlannerHow to Prepare a Room-by-Room Airflow Schedule
Lay out rooms, volumes, loads and target CFM in one table your HVAC contractor can check. Includes a three-room worked schedule and the data each column needs.
Open the Room-by-Room CFM PlannerRoof Pitch, Rise, Span, and Truss Spacing Explained
Definitions and formulas for roof pitch, rise, run, span, sloping length, overhang and truss count, with a labelled worked example. Geometry only — not structural design.
Open the Roof Truss Geometry CalculatorAsphalt Installation, Resurfacing, and Sealing: Cost Differences
What each asphalt driveway job includes, which line items drive cost, how to compare quotes fairly and why a national average isn't a local price.
Open the Asphalt Driveway Cost PlannerHow to Estimate Driveway Sealer Quantity
Calculate gallons and pails of driveway sealer from your product's label coverage and number of coats, with a worked example and tips for porous surfaces.
Open the Driveway Sealing CalculatorRunning Watts vs Starting Watts for Generator Planning
How running, additional starting and total starting watts relate, how to add them without double counting, and the generator safety rules that matter most.
Open the Generator Wattage Planner