diff --git a/README.md b/README.md
index 50e981e..54cfadc 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,3 @@
 # go-eta
- ETA calculator for Go
+
+ETA calculator for Go.
diff --git a/eta.go b/eta.go
new file mode 100644
index 0000000..2840476
--- /dev/null
+++ b/eta.go
@@ -0,0 +1,190 @@
+package eta
+
+import (
+	"sync"
+	"time"
+)
+
+// EtaCalculator represents ETA calculator
+type EtaCalculator struct {
+	startTime time.Time
+	processed int
+
+	// Expected processing count
+	TotalCount int
+
+	periodDuration   time.Duration
+	currentPeriod    time.Time
+	currentProcessed int
+	stats            []int
+
+	mu sync.RWMutex
+}
+
+// New return new ETA calculator
+func New(periodDuration time.Duration, totalCount int) *EtaCalculator {
+	now := time.Now()
+
+	etaCalc := &EtaCalculator{
+		startTime:      now,
+		TotalCount:     totalCount,
+		currentPeriod:  now.Truncate(periodDuration),
+		periodDuration: periodDuration}
+
+	return etaCalc
+}
+
+// Increment increments processing count
+func (ec *EtaCalculator) Increment(n int) {
+	if n <= 0 {
+		return
+	}
+
+	now := time.Now()
+
+	ec.mu.Lock()
+	defer ec.mu.Unlock()
+
+	ec.processed += n
+
+	// -------------------------------------------------------------------------
+	period := now.Truncate(ec.periodDuration)
+
+	if ec.currentPeriod == period {
+		ec.currentProcessed += n
+		return
+	} else {
+		ec.stats = append(ec.stats, ec.currentProcessed)
+		ec.currentProcessed = 0
+		ec.currentPeriod = period
+	}
+
+	if len(ec.stats) > 10 {
+		ec.stats = ec.stats[:10]
+	}
+}
+
+// Last returns ETA based on last period processing speed
+func (ec *EtaCalculator) Last() time.Time {
+	if ec.processed == 0 {
+		return time.Time{}
+	}
+
+	ec.mu.RLock()
+	defer ec.mu.RUnlock()
+
+	lastPeriodSpeed := ec.periodDuration / time.Duration(ec.stats[len(ec.stats)-1])
+
+	return time.Now().Add(lastPeriodSpeed * time.Duration(ec.TotalCount-ec.processed))
+}
+
+// Eta returns ETA based on total time and total processed items count
+func (ec *EtaCalculator) Eta() time.Time {
+	if ec.processed == 0 {
+		return time.Time{}
+	}
+
+	ec.mu.RLock()
+	defer ec.mu.RUnlock()
+
+	now := time.Now()
+	elapsedTime := now.Sub(ec.startTime)
+	avgSpeed := elapsedTime / time.Duration(ec.processed)
+
+	return now.Add(avgSpeed * time.Duration(ec.TotalCount-ec.processed))
+}
+
+// Average returns ETA based on average processing speed of last periods
+func (ec *EtaCalculator) Average() time.Time {
+	if len(ec.stats) == 0 {
+		return ec.Eta()
+	}
+
+	ec.mu.RLock()
+	defer ec.mu.RUnlock()
+
+	now := time.Now()
+
+	processed := ec.stats[len(ec.stats)-1]
+	startPeriod := ec.currentPeriod.Add(-ec.periodDuration)
+
+	for i := len(ec.stats) - 2; i >= 0; i-- {
+		processed += ec.stats[i]
+		startPeriod = startPeriod.Add(-ec.periodDuration)
+	}
+
+	if processed == 0 {
+		return time.Time{}
+	}
+
+	avgSpeed := ec.currentPeriod.Sub(startPeriod) / time.Duration(processed)
+
+	return now.Add(time.Duration(ec.TotalCount-ec.processed) * avgSpeed)
+}
+
+// Optimistic returns ETA based on detected maximum of processing speed
+func (ec *EtaCalculator) Optimistic() time.Time {
+	if len(ec.stats) == 0 {
+		return ec.Eta()
+	}
+
+	ec.mu.RLock()
+	defer ec.mu.RUnlock()
+
+	now := time.Now()
+
+	var maxSpeed time.Duration
+	if ec.stats[len(ec.stats)-1] > 0 {
+		maxSpeed = ec.periodDuration / time.Duration(ec.stats[len(ec.stats)-1])
+	} else {
+		maxSpeed = 0
+	}
+
+	for i := len(ec.stats) - 2; i >= 1; i-- {
+		if ec.stats[i-1] == 0 {
+			continue
+		}
+
+		newMaxSpeed := ec.periodDuration / time.Duration(ec.stats[i-1])
+		if newMaxSpeed < maxSpeed && newMaxSpeed > 0 {
+			maxSpeed = newMaxSpeed
+		}
+	}
+
+	return now.Add(time.Duration(ec.TotalCount-ec.processed) * maxSpeed)
+}
+
+// Pessimistic returns ETA based on detected minimum of processing speed
+func (ec *EtaCalculator) Pessimistic() time.Time {
+	if len(ec.stats) == 0 {
+		return ec.Eta()
+	}
+
+	ec.mu.RLock()
+	defer ec.mu.RUnlock()
+
+	now := time.Now()
+
+	var minSpeed time.Duration
+	if ec.stats[len(ec.stats)-1] > 0 {
+		minSpeed = ec.periodDuration / time.Duration(ec.stats[len(ec.stats)-1])
+	} else {
+		minSpeed = 0
+	}
+
+	nulPeriods := 0
+
+	for i := len(ec.stats) - 2; i >= 1; i-- {
+		if ec.stats[i-1] == 0 {
+			nulPeriods += 1
+			continue
+		}
+
+		newMinSpeed := ec.periodDuration / time.Duration(ec.stats[i-1])
+		if newMinSpeed > minSpeed {
+			minSpeed = newMinSpeed
+		}
+	}
+
+	return now.Add(time.Duration(ec.TotalCount-ec.processed) * minSpeed * time.Duration(1+nulPeriods))
+}
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..13ade62
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,3 @@
+module github.com/nxshock/go-eta
+
+go 1.18