I have got the design in Figma. I see that button has height as 40px.

How to set it corectly using padding + height?

If to set height: 40px and add padding it comes to more 40px by height.

>Solution :

Setting box-sizing:border-box; on your button or on all your elements (my advise) with * selector solves your problem.

*{
  box-sizing:border-box;
}

button{
 height:40px;
 padding : 0.5rem;
}

<button>My button</button>

To know more about box-sizing, I really recommend you to read here on MDN. And here is an overview:

box-sizing

The box-sizing CSS property sets how the total width and height of an element is calculated.

The box-sizing property can be used to adjust this behavior:

content-box gives you the default CSS box-sizing behavior. If you set an element’s width to 100 pixels, then the element’s content box will be 100 pixels wide, and the width of any border or padding will be added to the final rendered width, making the element wider than 100px.

border-box tells the browser to account for any border and padding in the values you specify for an element’s width and height. If you set an element’s width to 100 pixels, that 100 pixels will include any border or padding you added, and the content box will shrink to absorb that extra width. This typically makes it much easier to size elements. box-sizing: border-box is the default styling that browsers use for the <table>, <select>, and <button> elements, and for <input> elements whose type is radio, checkbox, reset, button, submit, color, or search.