public class ClassContainingCalculationIntensiveCode
{
    public double SomeCalculation()
    {
       // code omitted
    }

    // NOTE: Returning a new task object encapsulating the calculation is all that's needed.
    //       The consuming code needs to use async/await keywords.
    public Task<double> SomeCalculationAsync()
    {
       return Task.Factory.StartNew(() =>
       {
           return SomeCalculation();
       });
    }
}

public class ClassContainingCalculationIntensiveCode
{
    public double SomeCalculation()
    {
       // code omitted
    }

    public await Task<double> SomeCalculationAsync()
    {
       return await Task.Factory.StartNew(() =>
       {
           return SomeCalculation();
       });
    }
}

Make an existing calculation intensive method asynchronous.

Make asynchronous method

C#