diff --git a/pkg/iostreams/iostreams.go b/pkg/iostreams/iostreams.go
index a76ae1df9..b3fe9a4e5 100644
--- a/pkg/iostreams/iostreams.go
+++ b/pkg/iostreams/iostreams.go
@@ -2,6 +2,7 @@ package iostreams
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -41,6 +42,7 @@ type IOStreams struct {
 	stderrTTYOverride bool
 	stderrIsTTY       bool
 	termWidthOverride int
+	ttySize           func() (int, int, error)
 
 	pagerCommand string
 	pagerProcess *os.Process
@@ -273,7 +275,7 @@ func (s *IOStreams) ForceTerminal(spec string) {
 		return
 	}
 
-	ttyWidth, _, err := ttySize()
+	ttyWidth, _, err := s.ttySize()
 	if err != nil {
 		return
 	}
@@ -284,7 +286,6 @@ func (s *IOStreams) ForceTerminal(spec string) {
 			s.termWidthOverride = int(float64(s.termWidthOverride) * (float64(p) / 100))
 		}
 	}
-
 }
 
 func (s *IOStreams) ColorScheme() *ColorScheme {
@@ -325,6 +326,7 @@ func System() *IOStreams {
 		colorEnabled: EnvColorForced() || (!EnvColorDisabled() && stdoutIsTTY),
 		is256enabled: Is256ColorSupported(),
 		pagerCommand: os.Getenv("PAGER"),
+		ttySize:      ttySize,
 	}
 
 	if stdoutIsTTY && stderrIsTTY {
@@ -345,6 +347,9 @@ func Test() (*IOStreams, *bytes.Buffer, *bytes.Buffer, *bytes.Buffer) {
 		In:     ioutil.NopCloser(in),
 		Out:    out,
 		ErrOut: errOut,
+		ttySize: func() (int, int, error) {
+			return -1, -1, errors.New("ttySize not implemented in tests")
+		},
 	}, in, out, errOut
 }
 
@@ -359,6 +364,7 @@ func isCygwinTerminal(w io.Writer) bool {
 	return false
 }
 
+// terminalSize measures the viewport of the terminal that the output stream is connected to
 func terminalSize(w io.Writer) (int, int, error) {
 	if f, isFile := w.(*os.File); isFile {
 		return term.GetSize(int(f.Fd()))
diff --git a/pkg/iostreams/iostreams_test.go b/pkg/iostreams/iostreams_test.go
new file mode 100644
index 000000000..fc4ead08b
--- /dev/null
+++ b/pkg/iostreams/iostreams_test.go
@@ -0,0 +1,68 @@
+package iostreams
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestIOStreams_ForceTerminal(t *testing.T) {
+	tests := []struct {
+		name      string
+		iostreams *IOStreams
+		arg       string
+		wantTTY   bool
+		wantWidth int
+	}{
+		{
+			name:      "explicit width",
+			iostreams: &IOStreams{},
+			arg:       "72",
+			wantTTY:   true,
+			wantWidth: 72,
+		},
+		{
+			name: "measure width",
+			iostreams: &IOStreams{
+				ttySize: func() (int, int, error) {
+					return 72, 0, nil
+				},
+			},
+			arg:       "true",
+			wantTTY:   true,
+			wantWidth: 72,
+		},
+		{
+			name: "measure width fails",
+			iostreams: &IOStreams{
+				ttySize: func() (int, int, error) {
+					return -1, -1, errors.New("ttySize sabotage!")
+				},
+			},
+			arg:       "true",
+			wantTTY:   true,
+			wantWidth: 80,
+		},
+		{
+			name: "apply percentage",
+			iostreams: &IOStreams{
+				ttySize: func() (int, int, error) {
+					return 72, 0, nil
+				},
+			},
+			arg:       "50%",
+			wantTTY:   true,
+			wantWidth: 36,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			tt.iostreams.ForceTerminal(tt.arg)
+			if isTTY := tt.iostreams.IsStdoutTTY(); isTTY != tt.wantTTY {
+				t.Errorf("IOStreams.IsStdoutTTY() = %v, want %v", isTTY, tt.wantTTY)
+			}
+			if tw := tt.iostreams.TerminalWidth(); tw != tt.wantWidth {
+				t.Errorf("IOStreams.TerminalWidth() = %v, want %v", tw, tt.wantWidth)
+			}
+		})
+	}
+}
diff --git a/pkg/iostreams/tty_size.go b/pkg/iostreams/tty_size.go
index 8370fc916..767000ce7 100644
--- a/pkg/iostreams/tty_size.go
+++ b/pkg/iostreams/tty_size.go
@@ -8,6 +8,7 @@ import (
 	"golang.org/x/term"
 )
 
+// ttySize measures the size of the controlling terminal for the current process
 func ttySize() (int, int, error) {
 	f, err := os.Open("/dev/tty")
 	if err != nil {