ProCalcHubEvery Calculation Made Simple

ProCalcHub Blog

Practical guides to everyday calculations — loans and mortgages, margins and VAT, percentages, BMI, dates and unit conversions. Each guide explains the method, the formula and a worked example.

Calculator guides

Each guide explains how a calculator works, the formula behind it and a worked example.

Business & Finance

  • Loan Calculator — Work out the fixed monthly payment on a personal loan, car loan or any amortizing loan, and see how much of what you pay goes to interest over the life of the loan.
  • Margin Calculator — Find your gross margin and markup from cost and selling price, or work backwards from a target margin to the price you need to charge.
  • Mortgage Calculator — Estimate the full monthly cost of a home loan — principal and interest plus property tax, homeowners insurance and HOA dues — and see how the loan pays down over time.
  • ROI Calculator — Measure how much an investment gained or lost relative to its cost, and compare investments held for different lengths of time with annualized ROI.
  • VAT Calculator — Add VAT to a net price or extract it from a VAT-inclusive price. The same maths works for sales tax and GST at any rate.

HVAC & Energy

  • Air Filter Actual Size Calculator — A “20 × 25 × 4” filter is not 20 × 25 × 4 inches. Convert a nominal size to typical actual dimensions, work out the nominal size from a filter you measured, or check the clearance between a filter and its slot.
  • Dual Fuel Switchover Temperature Calculator — A dual fuel system should hand over from the heat pump to the furnace when the furnace becomes the cheaper source of heat. Enter your prices and the heat pump’s rated COP to estimate that economic balance point.
  • Furnace Filter Yearly Quantity Calculator — Plan a year of HVAC filters: count your systems and filter slots, choose the filter type and household conditions, and see how often to change them, how many to buy and what they cost.
  • Heat Pump vs Gas Furnace Cost Calculator — Estimate what it costs to heat your home with an electric heat pump versus a gas, propane or oil furnace, using your own energy prices and efficiencies — and find the electricity price at which the heat pump stops being cheaper.
  • HVAC Repair vs Replace Calculator — Weigh a repair quote against replacing the system. The calculator compares five years of repairs, energy and the value of a new unit’s remaining life, and checks the rules of thumb technicians often quote.

Electrical & Power

  • Detached Garage EV Charger Cost Calculator — When the panel is in the house and the car is in a detached garage, most of the cost is in getting power across the yard. Estimate the trench, conduit, wire size for the distance and an optional garage subpanel.
  • EV Charger Circuit Size Calculator — Enter your Level 2 charger’s output current and the wire run to see the breaker size, the minimum copper or aluminum conductor, voltage drop over the distance and how many miles of range an hour of charging adds.
  • EV Charger Installation Cost Calculator — Break a home EV charger installation into its parts — hardware, labor, the wire run from your panel, wall or trench work, permits and possible panel upgrades — and see a realistic price range to compare quotes against.
  • Generator Runtime Calculator — Find out how many hours or days a portable or standby generator can run on the fuel you have, how much it burns per hour at different loads, and what a day of backup power costs in fuel.
  • Whole House Generator Size Calculator — Pick the appliances you want to keep running during an outage and see the running load, the starting surge from motors like the air conditioner and well pump, and the standby generator size that covers both.

Plumbing & Water

  • Water Hardness Converter: GPG to PPM — Water tests, softener manuals and utility reports use different hardness units. Convert between grains per gallon, ppm (mg/L as calcium carbonate), German and French degrees and mmol/L, and see where your water sits on the hardness scale.
  • Water Heater Repair vs Replace Calculator — A failed thermostat, element or gas valve can often be fixed — a leaking tank cannot. Compare repairing your water heater with replacing it over five years, including energy savings from a more efficient model.
  • Water Softener Regeneration Calculator — See how many days your softener can run between regenerations for your hardness and household, and roughly how much salt and water those regenerations use each year.
  • Water Softener Size Calculator — Work out how many grains of hardness your household removes each day and which softener rating — 24,000, 32,000, 48,000 grains or more — can handle it with a sensible regeneration interval.

Home Improvement

  • Acoustic Slat Wall Panel Calculator — Enter your wall and the panel size to see how many slat wall panels you need, how many boxes to order, how wide the last column must be cut and how many spares you will have.
  • Appliance Repair vs Replace Calculator — An appliance repair quote arrives and the question is always the same: fix it or buy new? Compare the two over five years using the appliance’s age, typical lifespan, the repair and replacement prices and energy use.
  • Half Drop Wallpaper Calculator — Half-drop patterns shift every other strip down by half a repeat, which changes how many drops you get from each roll. See the drops per roll, the waste the offset causes and a strip-by-strip A/B cutting plan.
  • Pond Liner Size Calculator — Work out the liner dimensions for a garden or koi pond: the length and width plus twice the depth and an overlap around the edge — and the nearest stock liner size, underlayment and water volume.
  • Wallpaper Roll Calculator — Count wallpaper the way installers do: by drops (full-height strips) per roll, not by square footage. Enter your walls, the roll size and the pattern repeat to see how many rolls to buy and exactly where each drop is cut.

Moving & Storage

  • 5x10 vs 10x10 Storage Unit Calculator — Deciding between a 5×10 and a 10×10? Enter what you are storing to see how full each unit would be, whether long items fit the 5-foot width, and what the extra space in a 10×10 costs per month.
  • Junk Removal Cost Calculator — Turn a list of furniture, appliances, bags and debris into a junk removal price range: the truck load it fills, the disposal fees that apply to items like refrigerators and mattresses, and extras such as stairs.
  • Junk Removal Volume Calculator — Count the furniture, appliances, bags and debris you want gone and see the volume in cubic yards, how much of a junk removal truck it fills and which dumpster size would hold it.
  • Packing Tape Rolls Calculator — Moving or shipping in bulk? Enter how many boxes you are sealing, their size and how you tape them to see how many inches each box uses and how many rolls of packing tape to buy.
  • Poly Mailer Size Calculator — Enter a product’s dimensions and how much it squashes to find the smallest standard poly mailer it fits in — so you do not pay to ship air or split a seam with a mailer that is too tight.
  • Storage Unit Size Calculator — Count what you are storing — sofas, beds, appliances, boxes — and see its total volume, the smallest storage unit that fits it with room to pack, and how full each common unit size would be.

Commercial Operations

  • Commercial Cleaning Cost Calculator — Budget for office and commercial janitorial service. The calculator estimates the labor hours a cleaning crew needs for your space and turns them into per-visit, weekly and monthly price ranges you can compare with bids.
  • Commercial Ice Machine Size Calculator — Estimate how much ice your business uses on a busy day and the ice machine production rating to look for — from customers, seats, drinks, rooms or beds — including a margin for hot kitchens.
  • Commercial Kitchen Hood CFM Calculator — Estimate how much air a commercial kitchen hood needs to exhaust based on its style (wall canopy, island, backshelf), its length and the heaviest appliance under it — plus a starting point for makeup air and duct size.
  • Grease Trap Size Calculator — Size a hydromechanical grease trap for a commercial kitchen sink: enter the compartment dimensions and the calculator works out the drain flow rate in gallons per minute and the matching interceptor rating.
  • Medical Office Cleaning Cost Calculator — Medical offices take longer to clean than ordinary offices: exam rooms, high-touch surfaces and restrooms need disinfection every visit. Estimate the labor involved and the monthly cost of routine clinic cleaning.
  • Warehouse Cleaning Cost Calculator — Warehouses combine a little office space with a lot of open floor. Estimate the labor for offices, floor sweeping or scrubbing, restrooms and loading docks, and the resulting cost per visit, week and month.

Unit Conversion

  • Currency Converter — Convert between 30 major world currencies using the European Central Bank's published reference rates (latest: 2026-09-11).
  • Unit Converter — Convert between metric, imperial and US customary units for length, weight, temperature, area, volume, speed, time and digital storage — with the exact formula for every conversion.

Math & Algebra

  • Percentage Calculator — Solve the everyday percentage questions — what is X% of Y, X is what percent of Y, the percentage change between two values, and a value increased or decreased by a percentage — with every step shown.
  • Quadratic Equation Solver — Solve ax² + bx + c = 0 with the quadratic formula. Enter the three coefficients to get the roots — real or complex — along with the discriminant, vertex and every step.

Date & Time

  • Date Difference Calculator — Count the exact number of days between two dates and see the same span in years, months and days, weeks, business days and hours.

Health & Fitness

  • BMI Calculator — Calculate your body mass index from your height and weight, see which WHO category it falls in and the healthy weight range for your height.

Text & Data

  • Password Generator — Create strong, random passwords with the length and character types you need. Passwords are generated on your device using cryptographically secure randomness and never leave your browser.

Special Calculators

  • Party Supplies Calculator — Turn a guest count into a shopping list: how many plates, cups, napkins and utensils to buy for a snack party, buffet or served meal, plus ice and trash bags — and how many packs that means.
  • Tablecloth Size Calculator — Choose a table and how far you want the cloth to hang, and see the ideal tablecloth size, the closest standard linen and the drop it will actually give on every side.