Every calculator on gizmo421 is built in-house and runs entirely in your browser. No sign-ups, no server round-trips — just the math, done right, updated live as you type.
Basic Calculator
A fast calculator for everyday math — add, subtract, multiply, divide, and percentages, with full keyboard support.
open →Scientific Calculator
Trig functions, logarithms, powers, and factorials with a degree/radian toggle — results preview live as you type.
open →Mortgage Calculator
Estimate monthly mortgage payments with taxes, PMI, a principal-vs-interest breakdown, and a full month-by-month amortization schedule.
open →Concrete Calculator
Figure out how many cubic yards (or bags) of concrete a slab, footing, column, or tube form needs — in feet, inches, yards, or metric.
open →Time Duration Calculator
Find the exact duration between two times, or add and subtract hours and days from any date — timesheet math made easy.
open →Percentage Calculator
Percent of a number, “is what percent,” and percentage increase or decrease — any two boxes filled solves the third.
open →Age Calculator
Exact age in years, months, and days from a birthdate, with totals and a next-birthday countdown.
open →BMI Calculator
Body mass index in US or metric units, with your weight category and a healthy range for your height.
open →Date Calculator
Count days between two dates, or add and subtract days, weeks, months, and years from any date.
open →Unit Converter
Length, weight, temperature, volume, area, and speed — metric and US units converted live.
open →Tip Calculator
Tip, total, and per-person split for any bill — with quick 10–20% preset buttons.
open →Discount & Sales Tax Calculator
The real checkout price: sale price after a discount, sales tax, and the final total.
open →Loan Payoff Calculator
Months to pay off a loan, total interest, and how much an extra monthly payment saves.
open →Compound Interest Calculator
Watch savings grow: final balance, total invested, and interest earned — plus a year-by-year table.
open →Calorie Calculator
Daily calorie targets: BMR, maintenance (TDEE), and weight loss or gain goals, US or metric.
open →Aspect Ratio Calculator
Scale resolutions without distortion — 16:9, 4:3, 9:16 — and solve the missing dimension.
open →Sleep Calculator
Bedtimes and wake times built around 90-minute sleep cycles, so you wake between cycles instead of mid-dream.
open →Final Grade Calculator
Find the exact score you need on your final exam to hit your target grade — weighted averages, solved both directions.
open →Time Card Calculator
Add up a work week from clock-in and clock-out times — breaks, overnight shifts, overtime hours, and estimated pay.
open →Bandwidth Calculator
Convert megabits to megabytes and find real download times — the factor-of-eight mystery, solved for every file size.
open →GPA Calculator
Weighted GPA from grades and credit hours, plus exactly what you need next term to hit your goal.
open →Fuel Cost Calculator
Trip fuel and total cost from distance, MPG or L/100km, and gas price — with a carpool split.
open →Electricity Cost Calculator
What every appliance really costs per month — watts, hours, and your electric rate, added up live.
open →Speed, Distance & Time Calculator
Solve for whichever one you’re missing, with pace in minutes per km and per mile for runners.
open →More calculators are on the way — check back often.