diff --git a/cmd/cmount/fs.go b/cmd/cmount/fs.go
index 352e45d19..9f49c777a 100644
--- a/cmd/cmount/fs.go
+++ b/cmd/cmount/fs.go
@@ -268,25 +268,15 @@ func (fsys *FS) Releasedir(path string, fh uint64) (errc int) {
 func (fsys *FS) Statfs(path string, stat *fuse.Statfs_t) (errc int) {
 	defer log.Trace(path, "")("stat=%+v, errc=%d", stat, &errc)
 	const blockSize = 4096
-	const fsBlocks = (1 << 50) / blockSize
-	stat.Blocks = fsBlocks  // Total data blocks in file system.
-	stat.Bfree = fsBlocks   // Free blocks in file system.
-	stat.Bavail = fsBlocks  // Free blocks in file system if you're not root.
-	stat.Files = 1e9        // Total files in file system.
-	stat.Ffree = 1e9        // Free files in file system.
-	stat.Bsize = blockSize  // Block size
-	stat.Namemax = 255      // Maximum file name length?
-	stat.Frsize = blockSize // Fragment size, smallest addressable data size in the file system.
-	total, used, free := fsys.VFS.Statfs()
-	if total >= 0 {
-		stat.Blocks = uint64(total) / blockSize
-	}
-	if used >= 0 {
-		stat.Bfree = stat.Blocks - uint64(used)/blockSize
-	}
-	if free >= 0 {
-		stat.Bavail = uint64(free) / blockSize
-	}
+	total, _, free := fsys.VFS.Statfs()
+	stat.Blocks = uint64(total) / blockSize // Total data blocks in file system.
+	stat.Bfree = uint64(free) / blockSize   // Free blocks in file system.
+	stat.Bavail = stat.Bfree                // Free blocks in file system if you're not root.
+	stat.Files = 1e9                        // Total files in file system.
+	stat.Ffree = 1e9                        // Free files in file system.
+	stat.Bsize = blockSize                  // Block size
+	stat.Namemax = 255                      // Maximum file name length?
+	stat.Frsize = blockSize                 // Fragment size, smallest addressable data size in the file system.
 	mountlib.ClipBlocks(&stat.Blocks)
 	mountlib.ClipBlocks(&stat.Bfree)
 	mountlib.ClipBlocks(&stat.Bavail)
diff --git a/cmd/mount/fs.go b/cmd/mount/fs.go
index baa285fb6..016e97eac 100644
--- a/cmd/mount/fs.go
+++ b/cmd/mount/fs.go
@@ -54,25 +54,15 @@ var _ fusefs.FSStatfser = (*FS)(nil)
 func (f *FS) Statfs(ctx context.Context, req *fuse.StatfsRequest, resp *fuse.StatfsResponse) (err error) {
 	defer log.Trace("", "")("stat=%+v, err=%v", resp, &err)
 	const blockSize = 4096
-	const fsBlocks = (1 << 50) / blockSize
-	resp.Blocks = fsBlocks  // Total data blocks in file system.
-	resp.Bfree = fsBlocks   // Free blocks in file system.
-	resp.Bavail = fsBlocks  // Free blocks in file system if you're not root.
-	resp.Files = 1e9        // Total files in file system.
-	resp.Ffree = 1e9        // Free files in file system.
-	resp.Bsize = blockSize  // Block size
-	resp.Namelen = 255      // Maximum file name length?
-	resp.Frsize = blockSize // Fragment size, smallest addressable data size in the file system.
-	total, used, free := f.VFS.Statfs()
-	if total >= 0 {
-		resp.Blocks = uint64(total) / blockSize
-	}
-	if used >= 0 {
-		resp.Bfree = resp.Blocks - uint64(used)/blockSize
-	}
-	if free >= 0 {
-		resp.Bavail = uint64(free) / blockSize
-	}
+	total, _, free := f.VFS.Statfs()
+	resp.Blocks = uint64(total) / blockSize // Total data blocks in file system.
+	resp.Bfree = uint64(free) / blockSize   // Free blocks in file system.
+	resp.Bavail = resp.Bfree                // Free blocks in file system if you're not root.
+	resp.Files = 1e9                        // Total files in file system.
+	resp.Ffree = 1e9                        // Free files in file system.
+	resp.Bsize = blockSize                  // Block size
+	resp.Namelen = 255                      // Maximum file name length?
+	resp.Frsize = blockSize                 // Fragment size, smallest addressable data size in the file system.
 	mountlib.ClipBlocks(&resp.Blocks)
 	mountlib.ClipBlocks(&resp.Bfree)
 	mountlib.ClipBlocks(&resp.Bavail)
diff --git a/vfs/vfs.go b/vfs/vfs.go
index ccd10265c..2f35a0a8f 100644
--- a/vfs/vfs.go
+++ b/vfs/vfs.go
@@ -477,6 +477,37 @@ func (vfs *VFS) Rename(oldName, newName string) error {
 	return nil
 }
 
+// This works out the missing values from (total, used, free) using
+// unknownFree as the intended free space
+func fillInMissingSizes(total, used, free, unknownFree int64) (newTotal, newUsed, newFree int64) {
+	if total < 0 {
+		if free >= 0 {
+			total = free
+		} else {
+			total = unknownFree
+		}
+		if used >= 0 {
+			total += used
+		}
+	}
+	// total is now defined
+	if used < 0 {
+		if free >= 0 {
+			used = total - free
+		} else {
+			used = 0
+		}
+	}
+	// used is now defined
+	if free < 0 {
+		free = total - used
+	}
+	return total, used, free
+}
+
+// If the total size isn't known then we will aim for this many bytes free (1PB)
+const unknownFreeBytes = 1 << 50
+
 // Statfs returns into about the filing system if known
 //
 // The values will be -1 if they aren't known
@@ -488,10 +519,7 @@ func (vfs *VFS) Statfs() (total, used, free int64) {
 	defer vfs.usageMu.Unlock()
 	total, used, free = -1, -1, -1
 	doAbout := vfs.f.Features().About
-	if doAbout == nil {
-		return
-	}
-	if vfs.usageTime.IsZero() || time.Since(vfs.usageTime) >= vfs.Opt.DirCacheTime {
+	if doAbout != nil && (vfs.usageTime.IsZero() || time.Since(vfs.usageTime) >= vfs.Opt.DirCacheTime) {
 		var err error
 		vfs.usage, err = doAbout(context.TODO())
 		vfs.usageTime = time.Now()
@@ -510,15 +538,7 @@ func (vfs *VFS) Statfs() (total, used, free int64) {
 		if u.Used != nil {
 			used = *u.Used
 		}
-		if total < 0 && free >= 0 && used >= 0 {
-			total = free + used
-		}
-		if free < 0 && total >= 0 && used >= 0 {
-			free = total - used
-		}
-		if used < 0 && total >= 0 && free >= 0 {
-			used = total - free
-		}
 	}
+	total, used, free = fillInMissingSizes(total, used, free, unknownFreeBytes)
 	return
 }
diff --git a/vfs/vfs_test.go b/vfs/vfs_test.go
index ed3e2faf0..4216a3a89 100644
--- a/vfs/vfs_test.go
+++ b/vfs/vfs_test.go
@@ -4,6 +4,7 @@ package vfs
 
 import (
 	"context"
+	"fmt"
 	"io"
 	"os"
 	"testing"
@@ -311,3 +312,51 @@ func TestVFSStatfs(t *testing.T) {
 	assert.Equal(t, free, free2)
 	assert.Equal(t, oldTime, vfs.usageTime)
 }
+
+func TestFillInMissingSizes(t *testing.T) {
+	const unknownFree = 10
+	for _, test := range []struct {
+		total, free, used             int64
+		wantTotal, wantUsed, wantFree int64
+	}{
+		{
+			total: 20, free: 5, used: 15,
+			wantTotal: 20, wantFree: 5, wantUsed: 15,
+		},
+		{
+			total: 20, free: 5, used: -1,
+			wantTotal: 20, wantFree: 5, wantUsed: 15,
+		},
+		{
+			total: 20, free: -1, used: 15,
+			wantTotal: 20, wantFree: 5, wantUsed: 15,
+		},
+		{
+			total: 20, free: -1, used: -1,
+			wantTotal: 20, wantFree: 20, wantUsed: 0,
+		},
+		{
+			total: -1, free: 5, used: 15,
+			wantTotal: 20, wantFree: 5, wantUsed: 15,
+		},
+		{
+			total: -1, free: 15, used: -1,
+			wantTotal: 15, wantFree: 15, wantUsed: 0,
+		},
+		{
+			total: -1, free: -1, used: 15,
+			wantTotal: 25, wantFree: 10, wantUsed: 15,
+		},
+		{
+			total: -1, free: -1, used: -1,
+			wantTotal: 10, wantFree: 10, wantUsed: 0,
+		},
+	} {
+		t.Run(fmt.Sprintf("total=%d,free=%d,used=%d", test.total, test.free, test.used), func(t *testing.T) {
+			gotTotal, gotUsed, gotFree := fillInMissingSizes(test.total, test.used, test.free, unknownFree)
+			assert.Equal(t, test.wantTotal, gotTotal, "total")
+			assert.Equal(t, test.wantUsed, gotUsed, "used")
+			assert.Equal(t, test.wantFree, gotFree, "free")
+		})
+	}
+}