I started the project and I made this so far using Measurement<Unit>, but I don't not why my formated input Text shows the output. When I first run the app, the calculation seems to work, but when I change the units, it shows wrong answers (strange behaviour). Here is my code, I appreciate so much any help or guidence!

enum Temperature: String, Equatable, CaseIterable {
  case Celsius, Fahrenheit, Kelvin
}

struct ContentView: View {
  @State private var inputValue: Double = 0
  @State private var inputUnit: Temperature = .Celsius
  @State private var outputUnit: Temperature = .Fahrenheit
  @FocusState private var inputIsFocused: Bool

  var inputMeasurement: Measurement<UnitTemperature> {
    switch inputUnit {
    case .Celsius:
      return Measurement(value: inputValue, unit: UnitTemperature.celsius)
    case .Fahrenheit:
      return Measurement(value: inputValue, unit: UnitTemperature.fahrenheit)
    case .Kelvin:
      return Measurement(value: inputValue, unit: UnitTemperature.kelvin)
    }
  }

  var outputMeasurement: Measurement<UnitTemperature> {
    let output = inputMeasurement
    switch outputUnit {
    case .Celsius:
      return output.converted(to: .celsius)
    case .Fahrenheit:
      return output.converted(to: .fahrenheit)
    case .Kelvin:
      return output.converted(to: .kelvin)
    }
  }

  var body: some View {
    NavigationView {
      Form {
        Section {
          Picker("Input Unit", selection: $inputUnit) {
            ForEach(Temperature.allCases, id: \.self) {
              Text($0.rawValue)
            }
          }
          .pickerStyle(.automatic)

          TextField("Enter the amount", value: $inputValue, format: .number)
            .keyboardType(.decimalPad)
            .focused($inputIsFocused)
          Text(inputMeasurement.formatted())
        } header: {
          Text("Input Parameters")
        }

        Section {
          Picker("Output Unit", selection: $outputUnit) {
            ForEach(Temperature.allCases, id: \.self) {
              Text($0.rawValue)
            }
          }
          .pickerStyle(.automatic)

          Text(outputMeasurement.formatted())
        } header: {
          Text("Output Parameters")
        }
      }
      .navigationTitle("Unit Converter")
      .toolbar {
        ToolbarItemGroup(placement: .keyboard) {
          Spacer()
          Button("Done") {
            inputIsFocused = false
          }
        }
      }
    }
  }
}

If you want to use the new-style formatters, you can do this:

Text(outputMeasurement.formatted(.measurement(usage: .asProvided)))

Thank you guys! :)

Am I the only one that did plain math and if statements? Joke aside, at least I learned something from your approach!