How to get rid of if statements

This is a fake implementation of the bmr calculation which I use in my Ruby on Rails app.

Since the formula is using only metric units I had to somehow convert imperial units to the metric if imperial units are set to be default for the user.

I have come up with this code.

Personally I think there is a lot of code for this kind of small problem. On the contraty, using polymorphism would be over engineering. Ways to improve this code?

require 'ostruct'
require 'delegate'

module BmrCalculator
  class Calculator
    def call(sex = :male, measurement_unit = :metric)
      user = OpenStruct.new(sex: sex, weight: 2, height: 2, measurement_unit: measurement_unit)

      if measurement_unit == :imperial #dont like it
        user = ImperialToMetricDecorator.new(user)
      end

      BmrCalculator.new.bmr(user)
    end

    class BmrCalculator
      def bmr(user)
        if user.sex == :male
          puts user.inspect
          puts 1 * user.weight + 2 * user.height + 3 #this formula works only with metric units
        else
          puts user.inspect
          puts 6 * user.weight + 3 * user.height + 5 #this formula works only with metric units
        end
      end
    end
  end

  class ImperialToMetricDecorator < SimpleDelegator
    def height
      (super * 2.54)
    end

    def weight
      (super / 2.2)
    end
  end

  Calculator.new.call(:male, :metric)
end