C++ Unit Converter Project

Beginner•2–4 hours•functions, switch, constants

Convert length, weight, and temperature between metric and imperial units.

What you will build

Pick a category, pick a direction, type a value, get a converted result. It sounds like pure arithmetic, but the real lesson is organising many small conversion functions behind a clean menu — and handling temperature, which refuses to follow the simple multiply-by-a-factor pattern.

What you will learn

Writing one function per conversion
Using const for fixed conversion factors
Menu-driven program flow
Formatting floating-point output

Feature checklist

Length: metres ↔ feet, km ↔ miles
Weight: kilograms ↔ pounds
Temperature: Celsius ↔ Fahrenheit (formula, not factor)
Menu to choose category and direction
Rejects negative lengths/weights where they make no sense

Starter code

#include <iostream>

double cToF(double c) { return c * 9.0 / 5.0 + 32.0; }
double fToC(double f) { return (f - 32.0) * 5.0 / 9.0; }
double kmToMiles(double km) { return km * 0.621371; }
double kgToLb(double kg)    { return kg * 2.20462; }

int main() {
    std::cout << "1) C to F   2) F to C\n"
              << "3) km to miles   4) kg to lb\nChoice: ";
    int choice; double v;
    std::cin >> choice;
    std::cout << "Value: ";
    std::cin >> v;

    switch (choice) {
        case 1: std::cout << v << " C = " << cToF(v) << " F\n"; break;
        case 2: std::cout << v << " F = " << fToC(v) << " C\n"; break;
        case 3: std::cout << v << " km = " << kmToMiles(v) << " mi\n"; break;
        case 4: std::cout << v << " kg = " << kgToLb(v) << " lb\n"; break;
        default: std::cout << "Unknown choice.\n";
    }
}

Full walkthrough

Step 1: Write one small function per conversion

The starter's whole design is four one-line functions: cToF, fToC, kmToMiles, kgToLb. Each takes a double and returns a double, so the menu in main() never contains arithmetic — it only dispatches. When a conversion is wrong, you know the bug lives in exactly one line.

Step 2: Learn why temperature is the odd one out

Kilometres to miles is a pure factor (× 0.621371), but Celsius to Fahrenheit is c * 9.0 / 5.0 + 32.0 — a scale AND an offset, because the two scales do not share a zero. The checklist calls this out ('formula, not factor') because it is the one place a factor-table design breaks; write these two by hand and feel the difference.

double cToF(double c) { return c * 9.0 / 5.0 + 32.0; }
double fToC(double f) { return (f - 32.0) * 5.0 / 9.0; }

Step 3: Use named constants, not magic numbers

kgToLb multiplies by 2.20462 and kmToMiles by 0.621371. In a bigger version you would declare these as constexpr doubles with names like LB_PER_KG — the point is that a reader should never have to guess what 2.20462 means or hunt through code to update it.

Step 4: Build the menu with a switch

main() prints the four options exactly as the starter does, reads an int choice, then a double value, and the switch calls the matching function and prints 'value unit = result unit'. The default case catches any other number and prints 'Unknown choice.' — a menu without a default case fails silently, which users read as a crash.

Step 5: Mind the order of operations in your formulas

Write c * 9.0 / 5.0, not c * (9 / 5): in C++, 9 / 5 is integer division and equals 1, which would make 100 °C 'convert' to 132 °F. Using 9.0 forces floating-point division. This single bug is responsible for more broken converter homework than everything else combined.

Step 6: Reject nonsense values

The checklist asks you to refuse negative lengths and weights: after reading the value, if it is below zero for a km or kg conversion, print an error and stop. Temperatures are the exception — negative Celsius is perfectly real weather, so do not apply the rule blindly to every branch.

Step 7: Test against values you can verify

Boiling water: 100 °C must give exactly 212 °F. Freezing: 0 °C → 32 °F. The crossover point -40 °C → -40 °F is a beautiful test because both formulas must agree there. For distance, 5 km → 3.106855 mi, and 10 kg → 22.0462 lb.

Common mistakes

Writing 9 / 5 with integers and getting 1 — always make one side a double (9.0 / 5.0).
Swapping cToF and fToC in the switch cases, so the menu lies about the direction of conversion.
Applying the 'no negatives' rule to temperature and rejecting -10 °C, which is a legitimate input.

Compile and run

g++ -std=c++17 -o unit_converter unit_converter.cpp

Sample run

$ ./unit_converter
1) C to F   2) F to C
3) km to miles   4) kg to lb
Choice:
> 1
Value:
> 100
100 C = 212 F

$ ./unit_converter
1) C to F   2) F to C
3) km to miles   4) kg to lb
Choice:
> 3
Value:
> 5
5 km = 3.10686 mi

Try it in your browser

This project has a browser demo in the classic site. The demo is a browser re-implementation of the same logic, not the compiled C++ program: Length, weight, and temperature conversions — including the temperature formulas that don't follow a simple factor.

Open the browser demo in the classic site →

Related Beginner projects

← All 60 projects · Classic site (Learn & Sandbox)