diff --git a/builder/qemu/builder.go b/builder/qemu/builder.go
index 42d733792..6d85d2654 100644
--- a/builder/qemu/builder.go
+++ b/builder/qemu/builder.go
@@ -11,7 +11,9 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"regexp"
 	"runtime"
+	"strings"
 	"time"
 
 	"github.com/hashicorp/packer/common"
@@ -147,10 +149,11 @@ type Config struct {
 	// one of the other listed interfaces. Using the `scsi` interface under
 	// these circumstances will cause the build to fail.
 	DiskInterface string `mapstructure:"disk_interface" required:"false"`
-	// The size in bytes, suffixes of the first letter of common byte types
-	// like "k" or "K", "M" for megabytes, G for gigabytes, T for terabytes.
-	// Will create the of the hard disk of the VM. By default, this is
-	// `40960M` (40 GB).
+	// The size in bytes of the hard disk of the VM. Suffix with the first
+	// letter of common byte types. Use "k" or "K" for kilobytes, "M" for
+	// megabytes, G for gigabytes, and T for terabytes. If no value is provided
+	// for disk_size, Packer uses a default of `40960M` (40 GB). If a disk_size
+	// number is provided with no units, Packer will default to Megabytes.
 	DiskSize string `mapstructure:"disk_size" required:"false"`
 	// The cache mode to use for disk. Allowed values include any of
 	// `writethrough`, `writeback`, `none`, `unsafe` or `directsync`. By
@@ -354,6 +357,23 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 
 	if b.config.DiskSize == "" || b.config.DiskSize == "0" {
 		b.config.DiskSize = "40960M"
+	} else {
+		// Make sure supplied disk size is valid
+		// (digits, plus an optional valid unit character). e.g. 5000, 40G, 1t
+		re := regexp.MustCompile(`^[\d]+(b|k|m|g|t){0,1}$`)
+		matched := re.MatchString(strings.ToLower(b.config.DiskSize))
+		if !matched {
+			errs = packer.MultiErrorAppend(errs, fmt.Errorf("Invalid disk size."))
+		} else {
+			// Okay, it's valid -- if it doesn't alreay have a suffix, then
+			// append "M" as the default unit.
+			re = regexp.MustCompile(`^[\d]+$`)
+			matched = re.MatchString(strings.ToLower(b.config.DiskSize))
+			if matched {
+				// Needs M added.
+				b.config.DiskSize = fmt.Sprintf("%sM", b.config.DiskSize)
+			}
+		}
 	}
 
 	if b.config.DiskCache == "" {
@@ -457,7 +477,6 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	if b.config.ISOSkipCache {
 		b.config.ISOChecksumType = "none"
 	}
-
 	isoWarnings, isoErrs := b.config.ISOConfig.Prepare(&b.config.ctx)
 	warnings = append(warnings, isoWarnings...)
 	errs = packer.MultiErrorAppend(errs, isoErrs...)
diff --git a/builder/qemu/builder_test.go b/builder/qemu/builder_test.go
index 15f3db8b3..4616ed3e2 100644
--- a/builder/qemu/builder_test.go
+++ b/builder/qemu/builder_test.go
@@ -157,34 +157,38 @@ func TestBuilderPrepare_DiskCompaction(t *testing.T) {
 }
 
 func TestBuilderPrepare_DiskSize(t *testing.T) {
-	var b Builder
-	config := testConfig()
-
-	delete(config, "disk_size")
-	warns, err := b.Prepare(config)
-	if len(warns) > 0 {
-		t.Fatalf("bad: %#v", warns)
-	}
-	if err != nil {
-		t.Fatalf("bad err: %s", err)
-	}
-
-	if b.config.DiskSize != "40960M" {
-		t.Fatalf("bad size: %s", b.config.DiskSize)
-	}
-
-	config["disk_size"] = "60000M"
-	b = Builder{}
-	warns, err = b.Prepare(config)
-	if len(warns) > 0 {
-		t.Fatalf("bad: %#v", warns)
-	}
-	if err != nil {
-		t.Fatalf("should not have error: %s", err)
-	}
-
-	if b.config.DiskSize != "60000M" {
-		t.Fatalf("bad size: %s", b.config.DiskSize)
+	type testcase struct {
+		InputSize   string
+		OutputSize  string
+		ErrExpected bool
+	}
+
+	testCases := []testcase{
+		testcase{"", "40960M", false},       // not provided
+		testcase{"12345", "12345M", false},  // no unit given, defaults to M
+		testcase{"12345x", "12345x", true},  // invalid unit
+		testcase{"12345T", "12345T", false}, // terabytes
+		testcase{"12345b", "12345b", false}, // bytes get preserved when set.
+		testcase{"60000M", "60000M", false}, // Original test case
+	}
+	for _, tc := range testCases {
+		// Set input disk size
+		var b Builder
+		config := testConfig()
+		delete(config, "disk_size")
+		config["disk_size"] = tc.InputSize
+
+		warns, err := b.Prepare(config)
+		if len(warns) > 0 {
+			t.Fatalf("bad: %#v", warns)
+		}
+		if (err == nil) == tc.ErrExpected {
+			t.Fatalf("bad: error when providing disk size %s; Err expected: %t; err recieved: %v", tc.InputSize, tc.ErrExpected, err)
+		}
+
+		if b.config.DiskSize != tc.OutputSize {
+			t.Fatalf("bad size: received: %s but expected %s", b.config.DiskSize, tc.OutputSize)
+		}
 	}
 }
 
diff --git a/website/source/partials/builder/qemu/_Config-not-required.html.md b/website/source/partials/builder/qemu/_Config-not-required.html.md
index b0b3922cc..7c4b56b02 100644
--- a/website/source/partials/builder/qemu/_Config-not-required.html.md
+++ b/website/source/partials/builder/qemu/_Config-not-required.html.md
@@ -47,10 +47,11 @@
     one of the other listed interfaces. Using the `scsi` interface under
     these circumstances will cause the build to fail.
     
--   `disk_size` (string) - The size in bytes, suffixes of the first letter of common byte types
-    like "k" or "K", "M" for megabytes, G for gigabytes, T for terabytes.
-    Will create the of the hard disk of the VM. By default, this is
-    `40960M` (40 GB).
+-   `disk_size` (string) - The size in bytes of the hard disk of the VM. Suffix with the first
+    letter of common byte types. Use "k" or "K" for kilobytes, "M" for
+    megabytes, G for gigabytes, and T for terabytes. If no value is provided
+    for disk_size, Packer uses a default of `40960M` (40 GB). If a disk_size
+    number is provided with no units, Packer will default to Megabytes.
     
 -   `disk_cache` (string) - The cache mode to use for disk. Allowed values include any of
     `writethrough`, `writeback`, `none`, `unsafe` or `directsync`. By