Unsure what size you are? To ensure the right fit every time, all you need is a soft measuring tape. Start by measuring your waist circumference (this is just above your hips). Take the size that is within two centimetres of the measurement, and if in between sizes always go up to the next size.
Next to ensure the length is what you want, measure from the waist (just above your hips) down the leg this will give you the outside leg measurement. Lastly to ensure it wont be too tight around your thigh, measure the circumference and ensure it is not more then the leg opening.
These measurements will help ensure the perfect fit.
Generally the fit is smaller to size, so if you are still unsure what size, go up to the next size and it will still be fitted.