diff --git a/mem/mem.go b/mem/mem.go
index 67f8741e7c94ac25cff4837fd03b3a0c52ddbc08..f77c6f01f762cfe67617bb2074506df10bf38703 100644
--- a/mem/mem.go
+++ b/mem/mem.go
@@ -4,18 +4,44 @@ import (
 	"encoding/json"
 )
 
+// Memory usage statistics. Total, Available and Used contain numbers of bytes
+// for human consumption.
+//
+// The other fields in this struct contain kernel specific values.
 type VirtualMemoryStat struct {
-	Total       uint64  `json:"total"`
-	Available   uint64  `json:"available"`
-	Used        uint64  `json:"used"`
+	// Total amount of RAM on this system
+	Total uint64 `json:"total"`
+
+	// RAM available for programs to allocate
+	//
+	// This value is computed from the kernel specific values.
+	Available uint64 `json:"available"`
+
+	// RAM used by programs
+	//
+	// This value is computed from the kernel specific values.
+	Used uint64 `json:"used"`
+
+	// Percentage of RAM used by programs
+	//
+	// This value is computed from the kernel specific values.
 	UsedPercent float64 `json:"used_percent"`
-	Free        uint64  `json:"free"`
-	Active      uint64  `json:"active"`
-	Inactive    uint64  `json:"inactive"`
-	Buffers     uint64  `json:"buffers"`
-	Cached      uint64  `json:"cached"`
-	Wired       uint64  `json:"wired"`
-	Shared      uint64  `json:"shared"`
+
+	// This is the kernel's notion of free memory; RAM chips whose bits nobody
+	// cares about the value of right now. For a human consumable number,
+	// Available is what you really want.
+	Free uint64 `json:"free"`
+
+	// OS X / BSD specific numbers:
+	// http://www.macyourself.com/2010/02/17/what-is-free-wired-active-and-inactive-system-memory-ram/
+	Active   uint64 `json:"active"`
+	Inactive uint64 `json:"inactive"`
+	Wired    uint64 `json:"wired"`
+
+	// Linux specific numbers
+	// https://www.centos.org/docs/5/html/5.1/Deployment_Guide/s2-proc-meminfo.html
+	Buffers uint64 `json:"buffers"`
+	Cached  uint64 `json:"cached"`
 }
 
 type SwapMemoryStat struct {
diff --git a/mem/mem_test.go b/mem/mem_test.go
index 28693574a5aada2f78a0baace472951698601316..770b6f315f37cdc360ff4f79ef66964460ba9231 100644
--- a/mem/mem_test.go
+++ b/mem/mem_test.go
@@ -3,6 +3,8 @@ package mem
 import (
 	"fmt"
 	"testing"
+
+	"github.com/stretchr/testify/assert"
 )
 
 func TestVirtual_memory(t *testing.T) {
@@ -14,6 +16,21 @@ func TestVirtual_memory(t *testing.T) {
 	if v == empty {
 		t.Errorf("error %v", v)
 	}
+
+	assert.True(t, v.Total > 0)
+	assert.True(t, v.Available > 0)
+	assert.True(t, v.Used > 0)
+
+	assert.Equal(t, v.Total, v.Available+v.Used,
+		"Total should be computable from available + used: %v", v)
+
+	assert.True(t, v.Free > 0)
+	assert.True(t, v.Available > v.Free,
+		"Free should be a subset of Available: %v", v)
+
+	assert.InDelta(t, v.UsedPercent,
+		100*float64(v.Used)/float64(v.Total), 0.1,
+		"UsedPercent should be how many percent of Total is Used: %v", v)
 }
 
 func TestSwap_memory(t *testing.T) {
@@ -35,7 +52,7 @@ func TestVirtualMemoryStat_String(t *testing.T) {
 		UsedPercent: 30.1,
 		Free:        40,
 	}
-	e := `{"total":10,"available":20,"used":30,"used_percent":30.1,"free":40,"active":0,"inactive":0,"buffers":0,"cached":0,"wired":0,"shared":0}`
+	e := `{"total":10,"available":20,"used":30,"used_percent":30.1,"free":40,"active":0,"inactive":0,"wired":0,"buffers":0,"cached":0}`
 	if e != fmt.Sprintf("%v", v) {
 		t.Errorf("VirtualMemoryStat string is invalid: %v", v)
 	}