A 36-inch men's waist generally converts to a women's size 18 in standard US sizing. The conversion is based on actual waist measurement: women's size 18 jeans typically have a 35.5–36.5 inch waist.
Quick reference: men's 30 ≈ women's 10, men's 32 ≈ women's 14, men's 34 ≈ women's 16, men's 36 ≈ women's 18, men's 38 ≈ women's 20.
These conversions are approximate because women's pants are cut differently. Even at the same waist measurement, women's jeans have more curve through the hip and a shorter rise, so the fit will not feel identical.
If you're shopping unisex or men's pants and want a closer fit through the leg, try sizing down by one and going for a slim or skinny cut. If you want the relaxed men's silhouette in women's sizing, stick with the chart above.
Always measure your natural waist with a soft tape and compare to the brand's specific size chart — this is more accurate than trusting the label.