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_darwin.go b/mem/mem_darwin.go
index 33147bba795ddcd90cb6800f8afc3a5d1758cc6c..922b05cb4f60da498feb9ca7c1d8c33276c6974c 100644
--- a/mem/mem_darwin.go
+++ b/mem/mem_darwin.go
@@ -3,12 +3,29 @@
 package mem
 
 import (
+	"encoding/binary"
 	"strconv"
 	"strings"
+	"syscall"
 
 	"github.com/shirou/gopsutil/internal/common"
 )
 
+func getHwMemsize() (uint64, error) {
+	totalString, err := syscall.Sysctl("hw.memsize")
+	if err != nil {
+		return 0, err
+	}
+
+	// syscall.sysctl() helpfully assumes the result is a null-terminated string and
+	// removes the last byte of the result if it's 0 :/
+	totalString += "\x00"
+
+	total := uint64(binary.LittleEndian.Uint64([]byte(totalString)))
+
+	return total, nil
+}
+
 // SwapMemory returns swapinfo.
 func SwapMemory() (*SwapMemoryStat, error) {
 	var ret *SwapMemoryStat
diff --git a/mem/mem_darwin_cgo.go b/mem/mem_darwin_cgo.go
index 1d08528188e9c513dad79f5b33f81b6b4420e100..461631976e8b0e9eaf80b7a715d5a28dec8686fd 100644
--- a/mem/mem_darwin_cgo.go
+++ b/mem/mem_darwin_cgo.go
@@ -28,19 +28,20 @@ func VirtualMemory() (*VirtualMemoryStat, error) {
 		return nil, fmt.Errorf("host_statistics error=%d", status)
 	}
 
-	totalCount := vmstat.wire_count +
-		vmstat.active_count +
-		vmstat.inactive_count +
-		vmstat.free_count
+	pageSize := uint64(syscall.Getpagesize())
+	total, err := getHwMemsize()
+	if err != nil {
+		return nil, err
+	}
+	totalCount := C.natural_t(total / pageSize)
 
 	availableCount := vmstat.inactive_count + vmstat.free_count
 	usedPercent := 100 * float64(totalCount-availableCount) / float64(totalCount)
 
-	usedCount := totalCount - vmstat.free_count
+	usedCount := totalCount - availableCount
 
-	pageSize := uint64(syscall.Getpagesize())
 	return &VirtualMemoryStat{
-		Total:       pageSize * uint64(totalCount),
+		Total:       total,
 		Available:   pageSize * uint64(availableCount),
 		Used:        pageSize * uint64(usedCount),
 		UsedPercent: usedPercent,
diff --git a/mem/mem_darwin_nocgo.go b/mem/mem_darwin_nocgo.go
index d1bd4f657dc56a99d554c53bfb7abd2ab4679aeb..f3b085e8d194776829fc3ba1e8efeb929d9b4791 100644
--- a/mem/mem_darwin_nocgo.go
+++ b/mem/mem_darwin_nocgo.go
@@ -8,8 +8,6 @@ import (
 	"strconv"
 	"strings"
 	"syscall"
-
-	"github.com/shirou/gopsutil/internal/common"
 )
 
 // Runs vm_stat and returns Free and inactive pages
@@ -67,11 +65,7 @@ func parseVMStat(out string, vms *VirtualMemoryStat) error {
 func VirtualMemory() (*VirtualMemoryStat, error) {
 	ret := &VirtualMemoryStat{}
 
-	t, err := common.DoSysctrl("hw.memsize")
-	if err != nil {
-		return nil, err
-	}
-	total, err := strconv.ParseUint(t[0], 10, 64)
+	total, err := getHwMemsize()
 	if err != nil {
 		return nil, err
 	}
@@ -83,8 +77,8 @@ func VirtualMemory() (*VirtualMemoryStat, error) {
 	ret.Available = ret.Free + ret.Inactive
 	ret.Total = total
 
-	ret.Used = ret.Total - ret.Free
-	ret.UsedPercent = float64(ret.Total-ret.Available) / float64(ret.Total) * 100.0
+	ret.Used = ret.Total - ret.Available
+	ret.UsedPercent = 100 * float64(ret.Used) / float64(ret.Total)
 
 	return ret, nil
 }
diff --git a/mem/mem_darwin_test.go b/mem/mem_darwin_test.go
index 0f2ea3a93c626d58ab1386f6720697222e2a6269..6d9e6b89c490428d136c0c1819149cabefc5cf43 100644
--- a/mem/mem_darwin_test.go
+++ b/mem/mem_darwin_test.go
@@ -3,6 +3,9 @@
 package mem
 
 import (
+	"os/exec"
+	"strconv"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -12,13 +15,16 @@ func TestVirtualMemoryDarwin(t *testing.T) {
 	v, err := VirtualMemory()
 	assert.Nil(t, err)
 
-	assert.True(t, v.Total > 0)
-
-	assert.Equal(t, v.Total, v.Active+v.Inactive+v.Free+v.Wired)
+	outBytes, err := exec.Command("/usr/sbin/sysctl", "hw.memsize").Output()
+	assert.Nil(t, err)
+	outString := string(outBytes)
+	outString = strings.TrimSpace(outString)
+	outParts := strings.Split(outString, " ")
+	actualTotal, err := strconv.ParseInt(outParts[1], 10, 64)
+	assert.Nil(t, err)
+	assert.Equal(t, uint64(actualTotal), v.Total)
 
 	assert.True(t, v.Available > 0)
-	assert.True(t, v.Available < v.Total)
-	assert.Equal(t, v.Available, v.Total-v.Wired-v.Active, "%v", v)
 	assert.Equal(t, v.Available, v.Free+v.Inactive, "%v", v)
 
 	assert.True(t, v.Used > 0)
diff --git a/mem/mem_freebsd.go b/mem/mem_freebsd.go
index 7c64f105ac47a4d784d5607fc3ac86afd98b1eb9..808538d81511e8f3a113c951806227dce644fb91 100644
--- a/mem/mem_freebsd.go
+++ b/mem/mem_freebsd.go
@@ -79,8 +79,8 @@ func VirtualMemory() (*VirtualMemoryStat, error) {
 	}
 
 	ret.Available = ret.Inactive + ret.Cached + ret.Free
-	ret.Used = ret.Active + ret.Wired + ret.Cached
-	ret.UsedPercent = float64(ret.Total-ret.Available) / float64(ret.Total) * 100.0
+	ret.Used = ret.Total - ret.Available
+	ret.UsedPercent = float64(ret.Used) / float64(ret.Total) * 100.0
 
 	return ret, nil
 }
diff --git a/mem/mem_linux.go b/mem/mem_linux.go
index fc9226219d7fb4398d53797b4e5bc7ad63296b12..899da83dffb4c2fa7e17ebad997846300f1d020b 100644
--- a/mem/mem_linux.go
+++ b/mem/mem_linux.go
@@ -51,7 +51,7 @@ func VirtualMemory() (*VirtualMemoryStat, error) {
 	if !memavail {
 		ret.Available = ret.Free + ret.Buffers + ret.Cached
 	}
-	ret.Used = ret.Total - ret.Free
+	ret.Used = ret.Total - ret.Available
 	ret.UsedPercent = float64(ret.Total-ret.Available) / float64(ret.Total) * 100.0
 
 	return ret, nil
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)
 	}