CSS flex children gutters with margin

Issue

I have a gallery layout, and I want to separate every box with margin. The problem is the last box in every row is not align with the grid; there is still space on the right side, how can I do this without using padding? ( that will force me to add another wrapper div )

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}
ul {
  list-style-type: none;
}
.flex {
  display: -webkit-box;
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  -webkit-flex-wrap: wrap;
  -ms-flex-wrap: wrap;
  flex-wrap: wrap;
}
.align-center {
  -webkit-box-align: center;
  -webkit-align-items: center;
  -ms-flex-align: center;
  align-items: center;
}
.space-between {
  -webkit-box-pack: justify;
  -webkit-justify-content: space-between;
  -ms-flex-pack: justify;
  justify-content: space-between;
}
.container {
  max-width: 1000px;
  margin: 0 auto;
  height: 100%;
  padding: 0 1em;
  border: 1px solid;
}
nav {
  background: #464646;
  height: 70px;
}
.box {
  background: lightblue;
  padding: 1em;
  -webkit-box-flex: 0;
  -webkit-flex: 0 0 calc(50% - 1em);
  -ms-flex: 0 0 calc(50% - 1em);
  flex: 0 0 calc(50% - 1em);
  text-align: center;
  margin-right: 1em;
  margin-bottom: 1em;
}

<nav>
  <div class="container flex align-center space-between">

    <ul>
      <li>Logo</li>
    </ul>

    <ul>
      <li>Home</li>
    </ul>

  </div>
</nav>

<div class="container">

  <header>
    <h1>Header</h1>
  </header>

</div>

<div class="container">

  <div class="gallery flex">
    <div class="box">BOX 1</div>
    <div class="box">BOX 2</div>
    <div class="box">BOX 3</div>
  </div>

</div>

Note: I can't use flex grow: 1 because I don't always have two boxes in a row.

I can't use space between because the grid is dynamic so for example If the width of the box is 33.33% and you have five boxes, the second row will be with two boxes one on the left side and second on the right side.

Solution

Method # 01:

Add following css on parnet and child respectively (This method will work only when there are only 2 boxes in a row):

/* For Parent Element */
.flex {
  justify-content: space-between;
}

/* For Child Element */
.flex-child {
  flex: 0 0 calc(50% - 0.5em);
}

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}
ul {
  list-style-type: none;
}
.flex {
  display: -webkit-box;
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  -webkit-flex-wrap: wrap;
  -ms-flex-wrap: wrap;
  flex-wrap: wrap;
}
.align-center {
  -webkit-box-align: center;
  -webkit-align-items: center;
  -ms-flex-align: center;
  align-items: center;
}
.space-between {
  -webkit-box-pack: justify;
  -webkit-justify-content: space-between;
  -ms-flex-pack: justify;
  justify-content: space-between;
}
.container {
  max-width: 1000px;
  margin: 0 auto;
  height: 100%;
  padding: 0 1em;
  border: 1px solid;
}
nav {
  background: #464646;
  height: 70px;
}
.box {
  background: lightblue;
  padding: 1em;
  -webkit-box-flex: 0;
  -webkit-flex: 0 0 calc(50% - 0.5em);
  -ms-flex: 0 0 calc(50% - 0.5em);
  flex: 0 0 calc(50% - 0.5em);
  text-align: center;
  margin-bottom: 1em;
}

<nav>
  <div class="container flex align-center space-between">
    <ul>
      <li>Logo</li>
    </ul>
    <ul>
      <li>Home</li>
    </ul>
  </div>
</nav>
<div class="container">
  <header>
    <h1>Header</h1>
  </header>
</div>
<div class="container">
  <div class="gallery flex space-between">
    <div class="box">BOX 1</div>
    <div class="box">BOX 2</div>
    <div class="box">BOX 3</div>
    <div class="box">BOX 4</div>
    <div class="box">BOX 5</div>
    <div class="box">BOX 6</div>
    <div class="box">BOX 7</div>
    <div class="box">BOX 8</div>
    <div class="box">BOX 9</div>
  </div>
</div>

Method # 02:

1. Add margin on both sides of child elements.
2. Extend parent from left/right with same amount of negative margin.

Note: Add overflow: hidden on parent of .gallery if horizontal scroll appears.

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}
ul {
  list-style-type: none;
}
.flex {
  display: -webkit-box;
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  -webkit-flex-wrap: wrap;
  -ms-flex-wrap: wrap;
  flex-wrap: wrap;
  margin: 0 -0.5em;
}
.align-center {
  -webkit-box-align: center;
  -webkit-align-items: center;
  -ms-flex-align: center;
  align-items: center;
}
.space-between {
  -webkit-box-pack: justify;
  -webkit-justify-content: space-between;
  -ms-flex-pack: justify;
  justify-content: space-between;
}
.container {
  max-width: 1000px;
  margin: 0 auto;
  height: 100%;
  padding: 0 1em;
  border: 1px solid;
}
nav {
  background: #464646;
  height: 70px;
}
.box {
  background: lightblue;
  padding: 1em;
  -webkit-box-flex: 0;
  -webkit-flex: 0 0 calc(50% - 1em);
  -ms-flex: 0 0 calc(50% - 1em);
  flex: 0 0 calc(50% - 1em);
  text-align: center;
  margin: 0 0.5em 1em;
}

<nav>
  <div class="container flex align-center space-between">
    <ul>
      <li>Logo</li>
    </ul>
    <ul>
      <li>Home</li>
    </ul>
  </div>
</nav>
<div class="container">
  <header>
    <h1>Header</h1>
  </header>
</div>
<div class="container">
  <div class="gallery flex">
    <div class="box">BOX 1</div>
    <div class="box">BOX 2</div>
    <div class="box">BOX 3</div>
  </div>
</div>

Answered By - Mohammad Usman