diff --git a/README.md b/README.md
index 26666b8..e08b858 100644
--- a/README.md
+++ b/README.md
@@ -28,13 +28,15 @@ $ go build ./
 
 ```
 ./dcagdax --help
-usage: dcagdax --every=EVERY --usd=USD [<flags>]
+usage: dcagdax --every=EVERY [<flags>]
 
 Flags:
-  --help         Show context-sensitive help (also try --help-long and --help-man).
+  --help         Show context-sensitive help (also try --help-long and
+                 --help-man).
   --coin=BTC     Which coin you want to buy: BTC, LTC, or ETH (default 'BTC').
   --every=EVERY  How often to make purchases, e.g. 1h, 7d, 3w.
-  --usd=USD      How much USD to spend on each purchase.
+  --usd=USD      How much USD to spend on each purchase. If unspecified, the
+                 minimum purchase amount allowed will be used.
   --until=UNTIL  Stop executing trades after this date, e.g. 2017-12-31.
   --trade        Actually execute trades.
   --autofund     Automatically initiate ACH deposits.
diff --git a/main.go b/main.go
index 0e0001a..3b36999 100644
--- a/main.go
+++ b/main.go
@@ -26,8 +26,8 @@ var (
 
 	usd = kingpin.Flag(
 		"usd",
-		"How much USD to spend on each purchase.",
-	).Required().Float()
+		"How much USD to spend on each purchase. If unspecified, the minimum purchase amount allowed will be used.",
+	).Float()
 
 	until = registerDate(kingpin.Flag(
 		"until",
diff --git a/schedule.go b/schedule.go
index 834a3d4..ed50604 100644
--- a/schedule.go
+++ b/schedule.go
@@ -52,6 +52,10 @@ func newGdaxSchedule(
 		return nil, err
 	}
 
+	if schedule.usd == 0.0 {
+		schedule.usd = minimum + 0.1
+	}
+
 	if schedule.usd < minimum {
 		return nil, errors.New(fmt.Sprintf(
 			"GDAX's minimum %s trade amount is $%.02f, but you're trying to purchase $%f",
@@ -59,9 +63,17 @@ func newGdaxSchedule(
 		))
 	}
 
+	// GDAX has a limit of 8 decimal places.
+	schedule.usd = roundFloat(schedule.usd, 8)
+
 	return &schedule, nil
 }
 
+func roundFloat(f float64, places int) (float64) {
+    shift := math.Pow(10, float64(places))
+    return math.Floor(f * shift + .5) / shift;
+}
+
 // Sync initiates trades & funding with a DCA strategy.
 func (s *gdaxSchedule) Sync() error {
 
@@ -147,7 +159,7 @@ func (s *gdaxSchedule) minimumUSDPurchase() (float64, error) {
 
 	for _, p := range products {
 		if p.BaseCurrency == s.coin {
-			return p.BaseMinSize * ticker.Price, nil
+			return math.Max(p.BaseMinSize * ticker.Price, 1.0), nil
 		}
 	}